Euler's sum of powers conjecture

Euler's conjecture is a disproved conjecture in mathematics related to Fermat's last theorem. It was proposed by Leonhard Euler in 1769. It states that for all integers n and k greater than 1, if the sum of n kth powers of non-zero integers is itself a kth power, then n is greater than or equal to k.

In symbols, the conjecture falsely states that if

$\sum_{i=1}^{n} a_i^k = b^k$

where n > 1 and a₁, a₂, …, a_n, b are non-zero integers, then n ≥ k.

The conjecture represents an attempt to generalize Fermat's last theorem, which is the special case n = 2: if a₁^k + a₂^k = b^k, then 2 ≥ k.

Although the conjecture holds for the case k = 3 (which follows from Fermat's last theorem for the third powers), it was disproved for k = 4 and k = 5. It is unknown whether the conjecture fails or holds for any value k ≥ 6.

Background

Euler had an equality for four fourth powers 59⁴ + 158⁴ = 133⁴ + 134⁴; this however is not a counterexample because no term is isolated on one side of the equation. He also provided a complete solution to the four cubes problem as in Plato's number 3³ + 4³ + 5³ = 6³ or the taxicab number 1729.^[1][2] The general solution for:

$x_1^3+x_2^3=x_3^3+x_4^3$

is

$x_1 = 1-(a-3b)(a^2+3b^2), x_2 = (a+3b)(a^2+3b^2)-1$

$x_3 = (a+3b)-(a^2+3b^2)^2, x_4 = (a^2+3b^2)^2-(a-3b)$

where a and b are any integers.

Counterexamples

Euler's conjecture was disproven by L. J. Lander and T. R. Parkin in 1966 when, through a direct computer search on a CDC 6600, they found a counterexample for k = 5.^[3] A total of three primitive (that is, in which the summands do not all have a common factor) counterexamples are known:

27⁵ + 84⁵ + 110⁵ + 133⁵ = 144⁵ (Lander & Parkin, 1966),

(−220)⁵ + 5027⁵ + 6237⁵ + 14068⁵ = 14132⁵ (Scher & Seidl, 1996), and

55⁵ + 3183⁵ + 28969⁵ + 85282⁵ = 85359⁵ (Frye, 2004).

In 1986, Noam Elkies found a method to construct an infinite series of counterexamples for the k = 4 case.^[4] His smallest counterexample was

2682440⁴ + 15365639⁴ + 18796760⁴ = 20615673⁴.

A particular case of Elkies' solutions can be reduced to the identity^[5][6]

(85v² + 484v − 313)⁴ + (68v² − 586v + 10)⁴ + (2u)⁴ = (357v² − 204v + 363)⁴

where

u² = 22030 + 28849v − 56158v² + 36941v³ − 31790v⁴.

This is an elliptic curve with a rational point at v₁ = −31/467. From this initial rational point, one can compute an infinite collection of others. Substituting v₁ into the identity and removing common factors gives the numerical example cited above.

In 1988, Roger Frye found the smallest possible counterexample

95800⁴ + 217519⁴ + 414560⁴ = 422481⁴

for k = 4 by a direct computer search using techniques suggested by Elkies. This solution is the only one with values of the variables below 1,000,000.^[7]

Generalizations

In 1967, L. J. Lander, T. R. Parkin, and John Selfridge conjectured^[8] that if k > 3 and

$\sum_{i=1}^{n} a_i^k = \sum_{j=1}^{m} b_j^k$,

where a_i ≠ b_j are positive integers for all 1 ≤ i ≤ n and 1 ≤ j ≤ m, then m + n ≥ k. In the special case m = 1, the conjecture states that if

$\sum_{i=1}^{n} a_i^k = b^k$

(under the conditions given above) then n ≥ k − 1.

The special case may be described as the problem of giving a partition of a perfect power into few like powers. For k = 4, 5, 7, 8 and n = k or k − 1, there are many known solutions. Some of these are listed below. There are no solutions for k = 6 where b ≤ 272580.

k = 4

95800⁴ + 217519⁴ + 414560⁴ = 422481⁴ (R. Frye, 1988)^[4]

30⁴ + 120⁴ + 272⁴ + 315⁴ = 353⁴ (R. Norrie, 1911)^[8]

k = 5

27⁵ + 84⁵ + 110⁵ + 133⁵ = 144⁵ (Lander & Parkin, 1966)

19⁵ + 43⁵ + 46⁵ + 47⁵ + 67⁵ = 72⁵ (Lander, Parkin, Selfridge, smallest, 1967)^[8]

7⁵ + 43⁵ + 57⁵ + 80⁵ + 100⁵ = 107⁵ (Sastry, 1934, third smallest)^[8]

k = 7

127⁷ + 258⁷ + 266⁷ + 413⁷ + 430⁷ + 439⁷ + 525⁷ = 568⁷ (M. Dodrill, 1999)

k = 8

90⁸ + 223⁸ + 478⁸ + 524⁸ + 748⁸ + 1088⁸ + 1190⁸ + 1324⁸ = 1409⁸ (S. Chase, 2000)

See also

• Jacobi–Madden equation
• Prouhet–Tarry–Escott problem
• Beal's conjecture
• Pythagorean quadruple
• Generalized taxicab number
• Sums of powers, a list of related conjectures and theorems

References

1. William Dunham, ed. (2007). The Genius of Euler: Reflections on His Life and Work. The MAA. p. 220. ISBN 978-0-88385-558-4.
2. Titus Piezas III (2005). "Euler's Extended Conjecture".
3. L. J. Lander, T. R. Parkin; Parkin (1966). "Counterexample to Euler's conjecture on sums of like powers". Bull. Amer. Math. Soc. 72 (6): 1079. doi:10.1090/S0002-9904-1966-11654-3.
4. Noam Elkies (1988). "On A⁴ + B⁴ + C⁴ = D⁴" (PDF). Mathematics of Computation 51 (184): 825–835. doi:10.1090/S0025-5718-1988-0930224-9. JSTOR 2008781. MR 0930224.
5. "Elkies' a⁴+b⁴+c⁴ = d⁴".
6. "Sums of Three Fourth Powers".
7. Frye, Roger E. (1988), "Finding 95800⁴ + 217519⁴ + 414560⁴ = 422481⁴ on the Connection Machine", Proceedings of Supercomputing 88, Vol.II: Science and Applications, pp. 106–116, doi:10.1109/SUPERC.1988.74138.
8. L. J. Lander, T. R. Parkin, J. L. Selfridge; Parkin; Selfridge (1967). "A Survey of Equal Sums of Like Powers". Mathematics of Computation 21 (99): 446–459. doi:10.1090/S0025-5718-1967-0222008-0. JSTOR 2003249.

External links

• Tito Piezas III, A Collection of Algebraic Identities
• Jaroslaw Wroblewski, Equal Sums of Like Powers
• Ed Pegg Jr., Math Games, Power Sums
• James Waldby, A Table of Fifth Powers equal to a Fifth Power (2009)
• R. Gerbicz, J.-C. Meyrignac, U. Beckert, All solutions of the Diophantine equation a⁶ + b⁶ = c⁶ + d⁶ + e⁶ + f⁶ + g⁶ for a,b,c,d,e,f,g < 250000 found with a distributed Boinc project
• EulerNet: Computing Minimal Equal Sums Of Like Powers
• Weisstein, Eric W., "Euler's Sum of Powers Conjecture", MathWorld.
• Weisstein, Eric W., "Euler Quartic Conjecture", MathWorld.
• Weisstein, Eric W., "Diophantine Equation--4th Powers", MathWorld.
• Euler's Conjecture at library.thinkquest.org
• A simple explanation of Euler's Conjecture at Maths Is Good For You!