1,3

A002313 has the 4k+1 and 4k+2 primes.

Related to Stormer numbers.

Table of n, a(n) for n=1..75.

J. Todd, A problem on arc tangent relations, Amer. Math. Monthly, 56 (1949), 517-528.

a(n) = ((A002314(n-1))^2 + 1) / A002313(n).

a(3) = 2 because A002313(3)=13 and 13*2-1 = 5^2.

Cf. A002313, A002314, A005528.

Sequence in context: A222481 A200778 A132601 * A055972 A079168 A055205

Adjacent sequences: A047815 A047816 A047817 * A047819 A047820 A047821

nonn

N. J. A. Sloane.

Edited by Don Reble, Apr 13 2006

approved