Niven's theorem

In mathematics, Niven's theorem, named after Ivan Niven, states that the only rational values of θ in the interval 0 â‰¤ Î¸ â‰¤ 90 for which the sine of θ degrees is also a rational number are:[1]


\begin{align}
\sin 0^\circ & = 0, \\[10pt]
\sin 30^\circ & = \frac 12, \\[10pt]
\sin 90^\circ & = 1.
\end{align}

In radians, one would require that 0 â‰¤ x â‰¤ Ï€/2, that x/Ï€ be rational, and that sin x be rational. The conclusion is then that the only such values are sin 0 = 0, sin Ï€/6 = 1/2, and sin Ï€/2 = 1.

The theorem appears as Corollary 3.12 in Niven's book on irrational numbers.[2]

The theorem extends to the other trigonometric functions as well.[3]:p. 41 For rational values of θ, the only rational values of the sine or cosine are 0, ±1/2, and ±1; the only rational values of the secant or cosecant are ±1 and ±2; and the only rational values of the tangent or cotangent are 0 and ±1.

See also

References

  1. ↑ Schaumberger, Norman (1974). "A Classroom Theorem on Trigonometric Irrationalities". Two-Year College Mathematics Journal 5: 73–76. JSTOR 3026991.
  2. ↑ Niven, I. (1956). Irrational Numbers. Wiley. p. 41. MR 0080123.
  3. ↑ Niven, Ivan. Irrational Numbers, Carus Mathematical Monographs no. 11, 1956.

Further reading

External links

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