A047818 a(n) is the least number m such that A002313(n)*m - 1 is a square.

1, 1, 2, 1, 5, 1, 2, 10, 2, 10, 13, 5, 1, 10, 2, 10, 13, 5, 37, 2, 34, 1, 50, 34, 17, 1, 25, 13, 10, 65, 2, 41, 65, 53, 5, 29, 34, 10, 1, 50, 2, 74, 10, 26, 5, 85, 106, 5, 25, 13, 1, 10, 26, 2, 61, 37, 34, 17, 5, 1, 26, 13, 170, 10, 2, 5, 130, 58, 125, 106, 73, 130, 50, 26, 170

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn

Comments

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

Related to Stormer numbers.

Examples

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

Links

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

Crossrefs

Cf. A002313, A002314, A005528.

Formula

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

Extensions

Edited by Don Reble, Apr 13 2006