Pythagorean triple

The Pythagorean theorem: a2 + b2 = c2
Animation demonstrating the simplest case of the Pythagorean Triple: 32 + 42 = 52.

A Pythagorean triple consists of three positive integers a, b, and c, such that a2 + b2 = c2. Such a triple is commonly written (a, b, c), and a well-known example is (3, 4, 5). If (a, b, c) is a Pythagorean triple, then so is (ka, kb, kc) for any positive integer k. A primitive Pythagorean triple is one in which a, b and c are coprime. A right triangle whose sides form a Pythagorean triple is called a Pythagorean triangle.

The name is derived from the Pythagorean theorem, stating that every right triangle has side lengths satisfying the formula a2 + b2 = c2; thus, Pythagorean triples describe the three integer side lengths of a right triangle. However, right triangles with non-integer sides do not form Pythagorean triples. For instance, the triangle with sides a = b = 1 and c = √2 is right, but (1, 1, √2) is not a Pythagorean triple because √2 is not an integer. Moreover, 1 and √2 do not have an integer common multiple because √2 is irrational.

Examples


A scatter plot of the legs (a,b) of the Pythagorean triples with c less than 6000. Negative values are included to illustrate the parabolic patterns in the plot more clearly.

There are 16 primitive Pythagorean triples with c ≤ 100:

(3, 4, 5) (5, 12, 13) (8, 15, 17) (7, 24, 25)
(20, 21, 29) (12, 35, 37) (9, 40, 41) (28, 45, 53)
(11, 60, 61) (16, 63, 65) (33, 56, 65) (48, 55, 73)
(13, 84, 85) (36, 77, 85) (39, 80, 89) (65, 72, 97)

Note, for example, that (6, 8, 10) is not a primitive Pythagorean triple, as it is a multiple of (3, 4, 5). Each of these low-c points forms one of the more easily recognizable radiating lines in the scatter plot.

Additionally these are all the primitive Pythagorean triples with 100 < c ≤ 300:

(20, 99, 101) (60, 91, 109) (15, 112, 113) (44, 117, 125)
(88, 105, 137) (17, 144, 145) (24, 143, 145) (51, 140, 149)
(85, 132, 157) (119, 120, 169) (52, 165, 173) (19, 180, 181)
(57, 176, 185) (104, 153, 185) (95, 168, 193) (28, 195, 197)
(84, 187, 205) (133, 156, 205) (21, 220, 221) (140, 171, 221)
(60, 221, 229) (105, 208, 233) (120, 209, 241) (32, 255, 257)
(23, 264, 265) (96, 247, 265) (69, 260, 269) (115, 252, 277)
(160, 231, 281) (161, 240, 289) (68, 285, 293)

Generating a triple

Primitive Pythagorean triples shown as triangles on a graph
The primitive Pythagorean triples. The odd leg a is plotted on the horizontal axis, the even leg b on the vertical. The curvilinear grid is composed of curves of constant m - n and of constant m + n in Euclid's formula.
A plot of triples generated by Euclid's formula map out part of the z2 = x2 + y2 cone. A constant m or n traces out part of a parabola on the cone.

Euclid's formula[1] is a fundamental formula for generating Pythagorean triples given an arbitrary pair of positive integers m and n with m > n. The formula states that the integers

 a = m^2 - n^2 ,\ \, b = 2mn ,\ \, c = m^2 + n^2

form a Pythagorean triple. The triple generated by Euclid's formula is primitive if and only if m and n are coprime and mn is odd. If both m and n are odd, then a, b, and c will be even, and so the triple will not be primitive; however, dividing a, b, and c by 2 will yield a primitive triple if m and n are coprime.[2]

Every primitive triple arises from a unique pair of coprime numbers m, n, one of which is even. It follows that there are infinitely many primitive Pythagorean triples. This relationship of a, b and c to m and n from Euclid's formula is referenced throughout the rest of this article.

Despite generating all primitive triples, Euclid's formula does not produce all triples—for example, (9, 12, 15) cannot be generated using integer m and n. This can be remedied by inserting an additional parameter k to the formula. The following will generate all Pythagorean triples uniquely:

 a = k\cdot(m^2 - n^2)   ,\ \, b = k\cdot(2mn) ,\ \, c = k\cdot(m^2 + n^2)

where m, n, and k are positive integers with m > n, mn odd, and with m and n coprime.

That these formulas generate Pythagorean triples can be verified by expanding a2 + b2 using elementary algebra and verifying that the result coincides with c2. Since every Pythagorean triple can be divided through by some integer k to obtain a primitive triple, every triple can be generated uniquely by using the formula with m and n to generate its primitive counterpart and then multiplying through by k as in the last equation.

Many formulas for generating triples with particular properties have been developed since the time of Euclid.

Proof of Euclid's formula

That satisfaction of Euclid's formula by a, b, c is sufficient for the triangle to be Pythagorean is apparent from the fact that for positive integers m and n, m > n, the a, b, and c given by the formula are all positive integers, and from the fact that

 a^2+b^2 = (m^2 - n^2)^2 + (2mn)^2 = (m^2 + n^2)^2 = c^2.

A proof of the necessity that a, b, c be expressed by Euclid's formula for any primitive Pythagorean triple is as follows.[3] All such triples can be written as (a, b, c) where a2 + b2 = c2 and a, b, c are coprime. Thus a, b, c are pairwise coprime (if a prime number would divide two of them, it would divide also the third one). As a and b are coprime, one is odd, and one may suppose that it is a, by exchanging, if needed, a and b. This implies that b is even and c is odd (if b would be odd, c would be even, and c2 would be a multiple of 4, while a2 + b2 would be congruent to 2 modulo 4, as an odd square is congruent to 1 modulo 4).

From a^2+b^2=c^2 we obtain c^2-a^2=b^2 and hence (c-a)(c+a)=b^2. Then \tfrac{(c+a)}{b}=\tfrac{b}{(c-a)}. Since \tfrac{(c+a)}{b} is rational, we set it equal to \tfrac{m}{n} in lowest terms. Thus \tfrac{(c-a)}{b}=\tfrac{n}{m}, as being the reciprocal of \tfrac{(c+a)}{b}. Then solving

\frac{c}{b}+\frac{a}{b}=\frac{m}{n}, \quad \quad  \frac{c}{b}-\frac{a}{b}=\frac{n}{m}

for \tfrac{c}{b} and \tfrac{a}{b} gives

\frac{c}{b}=\frac{1}{2}\left(\frac{m}{n}+\frac{n}{m}\right)=\frac{m^2+n^2}{2mn}, \quad \quad \frac{a}{b}=\frac{1}{2}\left(\frac{m}{n}-\frac{n}{m}\right)=\frac{m^2-n^2}{2mn}.

As \tfrac{m}{n} is fully reduced, m and n are coprime, and they cannot be both even. If they were both odd, the numerator of \tfrac{m^2-n^2}{2mn} would be a multiple of 4 (because an odd square is congruent to 1 modulo 4), and the denominator 2mn would not be a multiple of 4, implying that a would be even. Thus one of m and n is odd and the other is even, and the numerators of the two fractions with denominator 2mn are odd. Thus these fractions are fully reduced (an odd prime dividing this denominator divides one of m and n and not the other; thus it does not divide m2 ± n2). One may thus equate numerators with numerators and denominators with denominators, giving Euclid's formula

 a = m^2 - n^2   ,\ \, b = 2mn ,\ \, c = m^2 + n^2 with m and n coprime and of opposite parities.

A longer but more commonplace proof is given in Maor (2007)[4] and Sierpiński (2003).[5]:4–7

Interpretation of parameters in Euclid's formula

Suppose the sides of a Pythagorean triangle are m^2-n^2, \,  2mn, and m^2+n^2, and suppose the angle between the leg m^2-n^2 and the hypotenuse m^2+n^2 is denoted as \theta. Then \tan{\theta}=\tfrac{2mn}{m^2-n^2} and \tan{\tfrac{\theta}{2}}=\tfrac{n}{m}.[6]

Elementary properties of primitive Pythagorean triples

General properties

The properties of a primitive Pythagorean triple (a, b, c) with a <  b < c (without specifying which of a or b is even and which is odd) include:

\tfrac{K}{s^2} = \tfrac{n(m-n)}{m(m+n)} = 1-\tfrac{c}{s}.

Special cases

In addition, special Pythagorean triples with certain additional properties can be guaranteed to exist:

Geometry of Euclid's formula

Points on a unit circle

3,4,5 maps to x,y point (4/5,3/5) on the unit circle
The rational points on a circle correspond, under stereographic projection, to the rational points of the line.

Euclid's formulae for a Pythagorean triple

a = 2mn,\quad b=m^2-n^2,\quad c=m^2+n^2

can be understood in terms of the geometry of rational number points on the unit circle (Trautman 1998). To motivate this, consider a right triangle with legs a and b, and hypotenuse c, where a, b, and c are positive integers. By the Pythagorean theorem, a2 + b2 = c2 or, dividing both sides by c2,

\left(\frac{a}{c}\right)^2 + \left(\frac{b}{c}\right)^2=1.

Geometrically, the point in the Cartesian plane with coordinates

x=\frac{a}{c},\quad y=\frac{b}{c}

is on the unit circle x2 + y2 = 1. In this equation, the coordinates x and y are given by rational numbers. Conversely, any point on the unit circle whose coordinates x, y are rational numbers gives rise to a primitive Pythagorean triple. Indeed, write x and y as fractions in lowest terms:

x=\frac{a}{c},\quad y=\frac{b}{c},

where the greatest common divisor of a, b, and c is 1. Then, since x and y are on the unit circle,

\left(\frac{a}{c}\right)^2 + \left(\frac{b}{c}\right)^2=1,

and so

a^2+b^2=c^2,

as claimed.

Tangent half-angle formula

There is a one-to-one correspondence between the rational numbers in (0, 2 − 1) and the primitive Pythagorean triples. The tangent half-angle formula for the angle α opposite the side of length a of a Pythagorean triangle gives

\tan(\alpha/2) = \frac{\sin(\alpha)}{\cos(\alpha) + 1} = \frac{a/c}{b/c + 1} = \frac{a}{b + c},

which will be a rational number. In particular, if 0 < a < b then tan(α/2) will be a rational number in the interval (0, 2 − 1) ≈ (0, 0.4142).

The process has an inverse. Whenever tan(α/2) is a rational number in (0, 2 − 1), the three expressions

\begin{align}
\sin(\alpha) &= \frac{2 \tan(\alpha/2)}{1 + \tan^2(\alpha/2)} \\
\cos(\alpha) &= \frac{1 - \tan^2(\alpha/2)}{1 + \tan^2(\alpha/2)} \\
\tan(\alpha) &= \frac{2 \tan(\alpha/2)}{1 - \tan^2(\alpha/2)}
\end{align}

will all be rational numbers. Writing tan(α) = a/b as a fraction in lowest terms and writing c = a2 + b2 gives the primitive Pythagorean triple (a, b, c) with a < b < c. Alternatively, writing sin(α) = a/c as a fraction in lowest terms and b = c2a2 gives the same primitive Pythagorean triple, as does writing cos(α) = b/c as a fraction in lowest terms, with a = c2b2.

Stereographic approach

Stereographic projection of the unit circle onto the x-axis. Given a point P on the unit circle, draw a line from P to the point N = (0, 1) (the north pole). The point P′ where the line intersects the x-axis is the stereographic projection of P. Inversely, starting with a point P′ on the x-axis, and drawing a line from P′ to N, the inverse stereographic projection is the point P where the line intersects the unit circle.

There is therefore a correspondence between points on the unit circle with rational coordinates and primitive Pythagorean triples. At this point, Euclid's formulae can be derived either by methods of trigonometry or equivalently by using the stereographic projection.

For the stereographic approach, suppose that P′ is a point on the x-axis with rational coordinates

P' = \left(\frac{m}{n},0\right).

Then, it can be shown by basic algebra that the point P has coordinates


P = \left(
 \frac{2\left(\frac{m}{n}\right)}{\left(\frac{m}{n}\right)^2+1},
 \frac{\left(\frac{m}{n}\right)^2-1}{\left(\frac{m}{n}\right)^2+1}
\right) =
\left(
 \frac{2mn}{m^2+n^2},
 \frac{m^2-n^2}{m^2+n^2}
\right).

This establishes that each rational point of the x-axis goes over to a rational point of the unit circle. The converse, that every rational point of the unit circle comes from such a point of the x-axis, follows by applying the inverse stereographic projection. Suppose that P(x, y) is a point of the unit circle with x and y rational numbers. Then the point P′ obtained by stereographic projection onto the x-axis has coordinates

\left(\frac{x}{1-y},0\right)

which is rational.

In terms of algebraic geometry, the algebraic variety of rational points on the unit circle is birational to the affine line over the rational numbers. The unit circle is thus called a rational curve, and it is this fact which enables an explicit parameterization of the (rational number) points on it by means of rational functions.

Pythagorean triangles in a 2D lattice

A 2D lattice is a regular array of isolated points where if any one point is chosen as the Cartesian origin (0, 0), then all the other points are at (x, y) where x and y range over all positive and negative integers. Any Pythagorean triangle with triple (a, b, c) can be drawn within a 2D lattice with vertices at coordinates (0, 0), (a, 0) and (0, b). The count of lattice points lying strictly within the bounds of the triangle is given by   \tfrac{(a-1)(b-1)-\gcd{(a,b)}+1}{2};[21] for primitive Pythagorean triples this interior lattice count is  \tfrac{(a-1)(b-1)}{2}. The area (by Pick's theorem equal to one less than the interior lattice count plus half the boundary lattice count) equals  \tfrac{ab}{2} .

The first occurrence of two primitive Pythagorean triples sharing the same area occurs with triangles with sides (20, 21, 29), (12, 35, 37) and common area 210 (sequence A093536 in OEIS). The first occurrence of two primitive Pythagorean triples sharing the same interior lattice count occurs with (18108, 252685, 253333), (28077, 162964, 165365) and interior lattice count 2287674594 (sequence A225760 in OEIS). Three primitive Pythagorean triples have been found sharing the same area: (4485,  5852,  7373), (3059, 8580, 9109), (1380, 19019, 19069) with area 13123110. As yet, no set of three primitive Pythagorean triples have been found sharing the same interior lattice count.

Spinors and the modular group

Pythagorean triples can likewise be encoded into a matrix of the form

X = \begin{bmatrix}
c+b & a\\
a & c-b
\end{bmatrix}.

A matrix of this form is symmetric. Furthermore, the determinant of X is

\det X = c^2 - a^2 - b^2\,

which is zero precisely when (a,b,c) is a Pythagorean triple. If X corresponds to a Pythagorean triple, then as a matrix it must have rank 1.

Since X is symmetric, it follows from a result in linear algebra that there is a column vector ξ = [m n]T such that the outer product

X = 2\begin{bmatrix}m\\n\end{bmatrix}[m\ n] = 2\xi\xi^T\,

 

 

 

 

(1)

holds, where the T denotes the matrix transpose. The vector ξ is called a spinor (for the Lorentz group SO(1, 2)). In abstract terms, the Euclid formula means that each primitive Pythagorean triple can be written as the outer product with itself of a spinor with integer entries, as in (1).

The modular group Γ is the set of 2×2 matrices with integer entries

A = \begin{bmatrix}\alpha&\beta\\ \gamma&\delta\end{bmatrix}

with determinant equal to one: αδ − βγ = 1. This set forms a group, since the inverse of a matrix in Γ is again in Γ, as is the product of two matrices in Γ. The modular group acts on the collection of all integer spinors. Furthermore, the group is transitive on the collection of integer spinors with relatively prime entries. For if [m n]T has relatively prime entries, then

\begin{bmatrix}m&-v\\n&u\end{bmatrix}\begin{bmatrix}1\\0\end{bmatrix} = \begin{bmatrix}m\\n\end{bmatrix}

where u and v are selected (by the Euclidean algorithm) so that mu + nv = 1.

By acting on the spinor ξ in (1), the action of Γ goes over to an action on Pythagorean triples, provided one allows for triples with possibly negative components. Thus if A is a matrix in Γ, then

2(A\xi)(A\xi)^T = A X A^T\,

 

 

 

 

(2)

gives rise to an action on the matrix X in (1). This does not give a well-defined action on primitive triples, since it may take a primitive triple to an imprimitive one. It is convenient at this point (per Trautman 1998) to call a triple (a,b,c) standard if c > 0 and either (a,b,c) are relatively prime or (a/2,b/2,c/2) are relatively prime with a/2 odd. If the spinor [m n]T has relatively prime entries, then the associated triple (a,b,c) determined by (1) is a standard triple. It follows that the action of the modular group is transitive on the set of standard triples.

Alternatively, restrict attention to those values of m and n for which m is odd and n is even. Let the subgroup Γ(2) of Γ be the kernel of the group homomorphism

\Gamma=\mathrm{SL}(2,\mathbf{Z})\to \mathrm{SL}(2,\mathbf{Z}_2)

where SL(2,Z2) is the special linear group over the finite field Z2 of integers modulo 2. Then Γ(2) is the group of unimodular transformations which preserve the parity of each entry. Thus if the first entry of ξ is odd and the second entry is even, then the same is true of Aξ for all A ∈ Γ(2). In fact, under the action (2), the group Γ(2) acts transitively on the collection of primitive Pythagorean triples (Alperin 2005).

The group Γ(2) is the free group whose generators are the matrices

U=\begin{bmatrix}1&2\\0&1\end{bmatrix},\qquad L=\begin{bmatrix}1&0\\2&1\end{bmatrix}.

Consequently, every primitive Pythagorean triple can be obtained in a unique way as a product of copies of the matrices U and L.

Parent/child relationships

By a result of Berggren (1934), all primitive Pythagorean triples can be generated from the (3, 4, 5) triangle by using the three linear transformations T1, T2, T3 below, where a, b, c are sides of a triple:

new side anew side bnew side c
T1: a − 2b + 2c 2ab + 2c2a − 2b + 3c
T2: a + 2b + 2c 2a + b + 2c2a + 2b + 3c
T3:a + 2b + 2c −2a + b + 2c −2a + 2b + 3c

If one begins with 3, 4, 5 then all other primitive triples will eventually be produced. In other words, every primitive triple will be a “parent” to 3 additional primitive triples. Starting from the initial node with a = 3, b = 4, and c = 5, the next generation of triples is

new side anew side bnew side c
3 − (2×4) + (2×5) = 5 (2×3) − 4 + (2×5) = 12 (2×3) − (2×4) + (3×5) = 13
3 + (2×4) + (2×5) = 21 (2×3) + 4 + (2×5) = 20 (2×3) + (2×4) + (3×5) = 29
−3 + (2×4) + (2×5) = 15 −(2×3) + 4 + (2×5) = 8 −(2×3) + (2×4) + (3×5) = 17

The linear transformations T1, T2, and T3 have a geometric interpretation in the language of quadratic forms. They are closely related to (but are not equal to) reflections generating the orthogonal group of x2 + y2z2 over the integers. A different set of three linear transformations is discussed in Pythagorean triples by use of matrices and linear transformations. For further discussion of parent-child relationships in triples, see: Pythagorean triple (Wolfram) and (Alperin 2005).

Relation to Gaussian integers

Alternatively, Euclid's formulae can be analyzed and proven using the Gaussian integers.[22] Gaussian integers are complex numbers of the form α = u + vi, where u and v are ordinary integers and i is the square root of negative one. The units of Gaussian integers are ±1 and ±i. The ordinary integers are called the rational integers and denoted as Z. The Gaussian integers are denoted as Z[i].. The right-hand side of the Pythagorean theorem may be factored in Gaussian integers:

c^2 = a^2+b^2 = (a+bi)\overline{(a+bi)} = (a+bi)(a-bi).

A primitive Pythagorean triple is one in which a and b are coprime, i.e., they share no prime factors in the integers. For such a triple, either a or b is even, and the other is odd; from this, it follows that c is also odd.

The two factors z := a + bi and z* := abi of a primitive Pythagorean triple each equal the square of a Gaussian integer. This can be proved using the property that every Gaussian integer can be factored uniquely into Gaussian primes up to units.[23] (This unique factorization follows from the fact that, roughly speaking, a version of the Euclidean algorithm can be defined on them.) The proof has three steps. First, if a and b share no prime factors in the integers, then they also share no prime factors in the Gaussian integers. (Assume a = gu and b = gv with Gaussian integers g, u and v and g not a unit. Then u and v lie on the same line through the origin. All Gaussian integers on such a line are integer multiples of some Gaussian integer h. But then the integer gh ≠ ±1 divides both a and b.) Second, it follows that z and z* likewise share no prime factors in the Gaussian integers. For if they did, then their common divisor δ would also divide z + z* = 2a and z  z* = 2ib. Since a and b are coprime, that implies that δ divides 2 = (1 + i)(1  i) = i(1  i)2. From the formula c2 = zz*, that in turn would imply that c is even, contrary to the hypothesis of a primitive Pythagorean triple. Third, since c2 is a square, every Gaussian prime in its factorization is doubled, i.e., appears an even number of times. Since z and z* share no prime factors, this doubling is also true for them. Hence, z and z* are squares.

Thus, the first factor can be written

a+bi = \varepsilon\left(m + ni \right)^2, \quad \varepsilon\in\{\pm 1, \pm i\}.

The real and imaginary parts of this equation give the two formulas:

\begin{cases}\varepsilon = +1, & \quad a = +\left( m^2 - n^2 \right),\quad b = +2mn; \\ \varepsilon = -1, & \quad a = -\left( m^2 - n^2 \right),\quad b = -2mn; \\ \varepsilon = +i, & \quad a = -2mn,\quad b = +\left( m^2 - n^2 \right); \\ \varepsilon = -i, &  \quad a = +2mn,\quad b = -\left( m^2 - n^2 \right).\end{cases}

For any primitive Pythagorean triple, there must be integers m and n such that these two equations are satisfied. Hence, every Pythagorean triple can be generated from some choice of these integers.

As perfect square Gaussian integers

If we consider the square of a Gaussian integer we get the following direct interpretation of Euclid's formulae as representing a perfect square Gaussian integers.

(m+ni)^2 = (m^2-n^2)+2mni.

Using the facts that the Gaussian integers are a Euclidean domain and that for a Gaussian integer p |p|^2 is always a square it is possible to show that a Pythagorean triples correspond to the square of a prime Gaussian integer if the hypotenuse is prime.

If the Gaussian integer is not prime then it is the product of two Gaussian integers p and q with |p|^2 and |q|^2 integers. Since magnitudes multiply in the Gaussian integers, the product must be |p||q|, which when squared to find a Pythagorean triple must be composite. The contrapositive completes the proof.

Distribution of triples

A scatter plot of the legs (a,b) of the first Pythagorean triples with a and b less than 4500.

There are a number of results on the distribution of Pythagorean triples. In the scatter plot, a number of obvious patterns are already apparent. Whenever the legs (a,b) of a primitive triple appear in the plot, all integer multiples of (a,b) must also appear in the plot, and this property produces the appearance of lines radiating from the origin in the diagram.

Within the scatter, there are sets of parabolic patterns with a high density of points and all their foci at the origin, opening up in all four directions. Different parabolas intersect at the axes and appear to reflect off the axis with an incidence angle of 45 degrees, with a third parabola entering in a perpendicular fashion. Within this quadrant, each arc centered on the origin shows that section of the parabola that lies between its tip and its intersection with its semi-latus rectum.

These patterns can be explained as follows. If a^2/4n is an integer, then (a, |n-a^2/4n|, n+a^2/4n) is a Pythagorean triple. (In fact every Pythagorean triple (a, b, c) can be written in this way with integer n, possibly after exchanging a and b, since n=(b+c)/2 and a and b cannot both be odd.) The Pythagorean triples thus lie on curves given by b = |n-a^2/4n|, that is, parabolas reflected at the a-axis, and the corresponding curves with a and b interchanged. If a is varied for a given n (i.e. on a given parabola), integer values of b occur relatively frequently if n is a square or a small multiple of a square. If several such values happen to lie close together, the corresponding parabolas approximately coincide, and the triples cluster in a narrow parabolic strip. For instance, 382 = 1444, 2 × 272 = 1458, 3 × 222 = 1452, 5 × 172 = 1445 and 10 × 122 = 1440; the corresponding parabolic strip around n ≈ 1450 is clearly visible in the scatter plot.

The angular properties described above follow immediately from the functional form of the parabolas. The parabolas are reflected at the a-axis at a = 2n, and the derivative of b with respect to a at this point is –1; hence the incidence angle is 45°. Since the clusters, like all triples, are repeated at integer multiples, the value 2n also corresponds to a cluster. The corresponding parabola intersects the b-axis at right angles at b = 2n, and hence its reflection upon interchange of a and b intersects the a-axis at right angles at a = 2n, precisely where the parabola for n is reflected at the a-axis. (The same is of course true for a and b interchanged.)

Albert Fässler and others provide insights into the significance of these parabolas in the context of conformal mappings.[24][25]

Special cases

The Platonic sequence

The case n = 1 of the more general construction of Pythagorean triples has been known for a long time. Proclus, in his commentary to the 47th Proposition of the first book of Euclid's Elements, describes it as follows:

Certain methods for the discovery of triangles of this kind are handed down, one which they refer to Plato, and another to Pythagoras. (The latter) starts from odd numbers. For it makes the odd number the smaller of the sides about the right angle; then it takes the square of it, subtracts unity and makes half the difference the greater of the sides about the right angle; lastly it adds unity to this and so forms the remaining side, the hypotenuse.
...For the method of Plato argues from even numbers. It takes the given even number and makes it one of the sides about the right angle; then, bisecting this number and squaring the half, it adds unity to the square to form the hypotenuse, and subtracts unity from the square to form the other side about the right angle. ... Thus it has formed the same triangle that which was obtained by the other method.

In equation form, this becomes:

a is odd (Pythagoras, c. 540 BC):

\text{side }a : \text{side }b = {a^2 - 1 \over 2} : \text{side }c = {a^2 + 1 \over 2}.

a is even (Plato, c. 380 BC):

\text{side }a : \text{side }b = \left({a \over 2}\right)^2 - 1 : \text{side }c = \left({a \over 2}\right)^2 + 1

It can be shown that all Pythagorean triples can be obtained, with appropriate rescaling, from the basic Platonic sequence (a, (a2 − 1)/2 and (a2 + 1)/2) by allowing a to take non-integer rational values. If a is replaced with the fraction m/n in the sequence, the result is equal to the 'standard' triple generator (2mn, m2n2,m2 + n2) after rescaling. It follows that every triple has a corresponding rational a value which can be used to generate a similar triangle (one with the same three angles and with sides in the same proportions as the original). For example, the Platonic equivalent of (56, 33, 65) is generated by a = m/n = 7/4 as (a, (a2 –1)/2, (a2+1)/2) = (56/32, 33/32, 65/32). The Platonic sequence itself can be derived by following the steps for 'splitting the square' described in Diophantus II.VIII.

The Jacobi-Madden equation

The equation,

a^4+b^4+c^4+d^4 = (a+b+c+d)^4

is equivalent to the special Pythagorean triple,

(a^2+ab+b^2)^2+(c^2+cd+d^2)^2 = ((a+b)^2+(a+b)(c+d)+(c+d)^2)^2

There is an infinite number of solutions to this equation as solving for the variables involves an elliptic curve. Small ones are,

a, b, c, d = -2634, 955, 1770, 5400
a, b, c, d = -31764, 7590, 27385, 48150

Equal sums of two squares

One way to generate solutions to a^2+b^2=c^2+d^2 is to parametrize a, b, c, d in terms of integers m, n, p, q as follows:[26]

(m^2+n^2)(p^2+q^2)=(mp-nq)^2+(np+mq)^2=(mp+nq)^2+(np-mq)^2.

Equal sums of two fourth powers

Given two sets of Pythagorean triples,

(a^2-b^2)^2+(2a b)^2 = (a^2+b^2)^2
(c^2-d^2)^2+(2c d)^2 = (c^2+d^2)^2

the problem of finding equal products of a non-hypotenuse side and the hypotenuse,

(a^2 -b^2)(a^2+b^2) = (c^2 -d^2)(c^2+d^2)

is easily seen to be equivalent to the equation,

a^4 -b^4 = c^4 -d^4

and was first solved by Euler as a, b, c, d = 133,59,158,134. Since he showed this is a rational point in an elliptic curve, then there is an infinite number of solutions. In fact, he also found a 7th degree polynomial parameterization.

Descartes' Circle Theorem

For the case of Descartes' circle theorem where all variables are squares,

2(a^4+b^4+c^4+d^4) = (a^2+b^2+c^2+d^2)^2

Euler showed this is equivalent to three simultaneous Pythagorean triples,

(2ab)^2+(2cd)^2 = (a^2+b^2-c^2-d^2)^2
(2ac)^2+(2bd)^2 = (a^2-b^2+c^2-d^2)^2
(2ad)^2+(2bc)^2 = (a^2-b^2-c^2+d^2)^2

There is also an infinite number of solutions, and for the special case when a+b=c, then the equation simplifies to,

4(a^2+a b+b^2) = d^2

with small solutions as a, b, c, d = 3, 5, 8, 14 and can be solved as binary quadratic forms.

Almost-isosceles Pythagorean triples

No Pythagorean triples are isosceles, because the ratio of the hypotenuse to either other side is 2, but 2 cannot be expressed as the ratio of 2 integers.

There are, however, right-angled triangles with integral sides for which the lengths of the non-hypotenuse sides differ by one, such as,

3^2+4^2 = 5^2
20^2+21^2 = 29^2

and an infinite number of others. They can be completely parameterized as,

(\tfrac{x-1}{2})^2+(\tfrac{x+1}{2})^2 = y^2

where {x, y} are the solutions to the Pell equation x^2-2y^2 = -1.

When it is the longer non-hypotenuse side and hypotenuse that differ by one, such as in

5^2+12^2 = 13^2
7^2+24^2 = 25^2

then the complete solution is

(2m+1)^2+(2m^2+2m)^2 = (2m^2+2m+1)^2

which also shows that all odd numbers (greater than 1) appear in a primitive Pythagorean triple.

Fibonacci numbers in Pythagorean triples

Starting with 5, every second Fibonacci number is the length of the hypotenuse of a right triangle with integer sides, or in other words, the largest number in a Pythagorean triple. The length of the longer leg of this triangle is equal to the sum of the three sides of the preceding triangle in this series of triangles, and the shorter leg is equal to the difference between the preceding bypassed Fibonacci number and the shorter leg of the preceding triangle.

Generalizations

There are several ways to generalize the concept of Pythagorean triples.

Pythagorean quadruple

Main article: Pythagorean quadruple

A set of four positive integers a, b, c and d such that a2 + b2+ c2 = d2 is called a Pythagorean quadruple. The simplest example is (1, 2, 2, 3), since 12 + 22 + 22 = 32. The next simplest (primitive) example is (2, 3, 6, 7), since 22 + 32 + 62 = 72.

All quadruples are given by the formula

(m^2+n^2-p^2-q^2)^2+(2mq+2np)^2+(2nq-2mp)^2=(m^2+n^2+p^2+q^2)^2.

Pythagorean n-tuple

Using the simple algebraic identity,

(x_1^2-x_0)^2 + (2x_1)^{2}x_0 = (x_1^2+x_0)^2

for arbitrary x0, x1, it is easy to prove that the square of the sum of n squares is itself the sum of n squares by letting x0 = x22 + x32 + ... + xn2 and then distributing terms.[27] One can see how Pythagorean triples and quadruples are just the particular cases x0 = x22 and x0 = x22 + x32, respectively, and so on for other n, with quintuples given by

(a^2-b^2-c^2-d^2)^2 + (2ab)^2 + (2ac)^2 + (2ad)^2 = (a^2+b^2+c^2+d^2)^2.

Since the sum F(k,m) of k consecutive squares beginning with m2 is given by the formula,[28]

F(k,m)=km(k-1+m)+\frac{k(k-1)(2k-1)}{6}

one may find values (k, m) so that F(k,m) is a square, such as one by Hirschhorn where the number of terms is itself a square,[29]

m=\tfrac{v^4-24v^2-25}{48},\; k=v^2,\; F(m,k)=\tfrac{v^5+47v}{48}

and v ≥ 5 is any integer not divisible by 2 or 3. For the smallest case v = 5, hence k = 25, this yields the well-known cannonball-stacking problem of Lucas,

0^2+1^2+2^2+\dots+24^2 = 70^2

a fact which is connected to the Leech lattice.

In addition, if in a Pythagorean n-tuple (n ≥ 4) all addends are consecutive except one, one can use the equation,[30]

F(k,m) + p^{2} = (p+1)^{2}

Since the second power of p cancels out, this is only linear and easily solved for as p=\tfrac{F(k,m)-1}{2} though k, m should be chosen so that p is an integer, with a small example being k = 5, m = 1 yielding,

1^2+2^2+3^2+4^2+5^2+27^2=28^2

Thus, one way of generating Pythagorean n-tuples is by using, for various x,[31]

x^2+(x+1)^2+\cdots +(x+q)^2+p^2=(p+1)^2,

where q = n–2 and where

p=\frac{(q+1)x^2+q(q+1)x+\frac{q(q+1)(2q+1)}{6} -1}{2}.

Fermat's Last Theorem

Main article: Fermat's Last Theorem

A generalization of the concept of Pythagorean triples is the search for triples of positive integers a, b, and c, such that an + bn = cn, for some n strictly greater than 2. Pierre de Fermat in 1637 claimed that no such triple exists, a claim that came to be known as Fermat's Last Theorem because it took longer than any other conjecture by Fermat to be proven or disproven. The first proof was given by Andrew Wiles in 1994.

n − 1 or n nth powers summing to an nth power

Another generalization is searching for sequences of n + 1 positive integers for which the nth power of the last is the sum of the nth powers of the previous terms. The smallest sequences for known values of n are:

For the n=3 case, in which x^3+y^3+z^3=w^3, called the Fermat cubic, a general formula exists giving all solutions.

A slightly different generalization allows the sum of (k + 1) nth powers to equal the sum of (n  k) nth powers. For example:

There can also exist n  1 positive integers whose nth powers sum to an nth power (though, by Fermat's last theorem, not for n = 3); these are counterexamples to Euler's sum of powers conjecture. The smallest known counterexamples are[32][33][11]

Heronian triangle triples

Main article: Heronian triangle

A Heronian triangle is commonly defined as one with integer sides whose area is also an integer, and we shall consider Heronian triangles with distinct integer sides. The lengths of the sides of such a triangle form a Heronian triple (a, b, c) provided a < b < c. Clearly, any Pythagorean triple is a Heronian triple, since in a Pythagorean triple at least one of the legs a, b must be even, so that the area ab/2 is an integer. Not every Heronian triple is a Pythagorean triple, however, as the example (4, 13, 15) with area 24 shows.

If (a, b, c) is a Heronian triple, so is (ma, mb, mc) where m is any positive integer greater than one. The Heronian triple (a, b, c) is primitive provided a, b, c are pairwise relatively prime (as with a Pythagorean triple). Here are a few of the simplest primitive Heronian triples that are not Pythagorean triples:

(4, 13, 15) with area 24
(3, 25, 26) with area 36
(7, 15, 20) with area 42
(6, 25, 29) with area 60
(11, 13, 20) with area 66
(13, 14, 15) with area 84
(13, 20, 21) with area 126

By Heron's formula, the extra condition for a triple of positive integers (a, b, c) with a < b < c to be Heronian is that

(a2 + b2 + c2)2 − 2(a4 + b4 + c4)

or equivalently

2(a2b2 + a2c2 + b2c2) − (a4 + b4 + c4)

be a nonzero perfect square divisible by 16.

Application to cryptography

Primitive Pythagorean triples have been used in cryptography as random sequences and for the generation of keys.[34]

See also

Notes

  1. Joyce, D. E. (June 1997), "Book X , Proposition XXIX", Euclid's Elements, Clark University
  2. Mitchell, Douglas W. (July 2001), "An Alternative Characterisation of All Primitive Pythagorean Triples", The Mathematical Gazette 85 (503): 273–5, doi:10.2307/3622017, JSTOR 3622017
  3. Beauregard, Raymond A.; Suryanarayan, E. R. (2000), "Parametric representation of primitive Pythagorean triples", in Nelsen, Roger B., Proofs Without Words: More Exercises in Visual Thinking II, Mathematical Association of America, p. 120, ISBN 978-0-88385-721-2, OCLC 807785075
  4. Maor, Eli, The Pythagorean Theorem, Princeton University Press, 2007: Appendix B.
  5. 1 2 3 4 5 Sierpiński, Wacław (2003), Pythagorean Triangles, Dover, ISBN 978-0-486-43278-6
  6. Houston, David (1993), "Pythagorean triples via double-angle formulas", in Nelsen, Roger B., Proofs Without Words: Exercises in Visual Thinking, Mathematical Association of America, p. 141, ISBN 978-0-88385-700-7, OCLC 29664480
  7. Posamentier, Alfred S. (2010), The Pythagorean Theorem: The Story of Its Power and Beauty, Prometheus Books, p. 156, ISBN 9781616141813.
  8. For the nonexistence of solutions where a and b are both square, originally proved by Fermat, see Koshy, Thomas (2002), Elementary Number Theory with Applications, Academic Press, p. 545, ISBN 9780124211711. For the other case, in which c is one of the squares, see Stillwell, John (1998), Numbers and Geometry, Undergraduate Texts in Mathematics, Springer, p. 133, ISBN 9780387982892.
  9. 1 2 3 Carmichael, R. D., 1914, "Diophantine analysis," in second half of R. D. Carmichael, The Theory of Numbers and Diophantine Analysis, Dover Publ., 1959.
  10. Sierpiński 2003, pp. 4–6
  11. 1 2 MacHale, Des; van den Bosch, Christian (March 2012), "Generalising a result about Pythagorean triples", Mathematical Gazette 96: 91–96
  12. Sally, Judith D. (2007), Roots to Research: A Vertical Development of Mathematical Problems, American Mathematical Society, pp. 74–75, ISBN 9780821872673.
  13. This follows immediately from the fact that one of a or b is divisible by four, together with the definition of congruent numbers as the areas of rational-sided right triangles. See e.g. Koblitz, Neal (1993), Introduction to Elliptic Curves and Modular Forms, Graduate Texts in Mathematics 97, Springer, p. 3, ISBN 9780387979663.
  14. Baragar, Arthur (2001), A Survey of Classical and Modern Geometries: With Computer Activities, Prentice Hall, Exercise 15.3, p. 301, ISBN 9780130143181
  15. 1 2 Bernhart, Frank R.; Price, H. Lee (2005). "Heron's formula, Descartes circles, and Pythagorean triangles". arXiv:math/0701624.
  16. Rosenberg, Steven; Spillane, Michael; Wulf, Daniel B. (May 2008), "Heron triangles and moduli spaces", Mathematics Teacher 101: 656–663
  17. 1 2 Yiu, Paul (2008), Heron triangles which cannot be decomposed into two integer right triangles (PDF), 41st Meeting of Florida Section of Mathematical Association of America, p. 17
  18. Pickover, Clifford A. (2009), "Pythagorean Theorem and Triangles", The Math Book, Sterling, p. 40, ISBN 1402757964
  19. Voles, Roger, "Integer solutions of a−2+b−2=d−2," Mathematical Gazette 83, July 1999, 269–271.
  20. Richinick, Jennifer, "The upside-down Pythagorean Theorem," Mathematical Gazette 92, July 2008, 313–317.
  21. Yiu, Paul. "RecreationalMathematics" (PDF). Course Notes, Chapter 2, page 110, Dept. of Mathematical Sciences, Florida Atlantic University (2003).
  22. Stillwell, John (2002), "6.6 Pythagorean Triples", Elements of Number Theory, Springer, pp. 110–2, ISBN 978-0-387-95587-2
  23. Gauss CF (1832), "Theoria residuorum biquadraticorum", Comm. Soc. Reg. Sci. Gött. Rec. 4. See also Werke, 2:67–148.
  24. 1988 Preprint See Figure 2 on page 3., later published as Fässler, Albert (June–July 1991), "Multiple Pythagorean number triples", American Mathematical Monthly 98 (6): 505–517, doi:10.2307/2324870, JSTOR 2324870
  25. Benito, Manuel; Varona, Juan L. (June 2002), "Pythagorean triangles with legs less than n", Journal of Computational and Applied Mathematics 143: 117–126, doi:10.1016/S0377-0427(01)00496-4 as PDF
  26. Nahin, Paul. An Imaginary Tale: The Story of \sqrt{-1}, pp. 25–26.
  27. "A Collection of Algebraic Identities: Sums of n Squares".
  28. "Sum of consecutive cubes equal a cube".
  29. Hirschhorn, Michael (November 2011), "When is the sum of consecutive squares a square?", The Mathematical Gazette 95: 511–2, ISSN 0025-5572, OCLC 819659848
  30. Goehl, John F. Jr. (May 2005), "Reader reflections", Mathematics Teacher 98 (9): 580
  31. Goehl, John F., Jr., "Triples, quartets, pentads", Mathematics Teacher 98, May 2005, p. 580.
  32. Kim, Scott (May 2002), "Bogglers", Discover: 82, The equation w4 + x4 + y4 = z4 is harder. In 1988, after 200 years of mathematicians' attempts to prove it impossible, Noam Elkies of Harvard found the counterexample, 2,682,4404 + 15,365,6394 + 18,796,7604 = 20,615,6734.
  33. Elkies, Noam (1988), "On A4 + B4 + C4 = D4", Mathematics of Computation 51: 825–835, doi:10.2307/2008781, MR 930224
  34. Kak, S. and Prabhu, M. Cryptographic applications of primitive Pythagorean triples. Cryptologia, 38:215-222, 2014.

References

External links

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