A083733 Pseudoprimes to bases 2 and 7.

561, 1105, 2465, 3277, 8321, 10585, 18721, 29341, 46657, 62745, 75361, 104653, 115921, 162401, 219781, 226801, 252601, 278545, 294409, 314821, 334153, 340561, 399001, 410041, 449065, 488881, 512461, 530881, 534061, 552721, 574561, 587861

Offset: 1

Author

Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), May 05 2003

Keywords

• nonn

Examples

			a(1)=561 since it is the first positive integer k(>1) which satisfies 2^(k-1) = 1 (mod k) and 7^(k-1) = 1 (mod k).
		

Links

• Amiram Eldar, Table of n, a(n) for n = 1..15498 (terms 1..138 from R. J. Mathar)
• F. Richman, Primality testing with Fermat's little theorem
• Index entries for sequences related to pseudoprimes

Crossrefs

Intersection of A005938 and A001567. - R. J. Mathar, Apr 05 2011

Programs

PARI

is(n)=!isprime(n)&&Mod(2,n)^(n-1)==1&&Mod(7,n)^(n-1)==1 \\ Charles R Greathouse IV, Apr 12 2012

Formula

a(n) = n-th positive integer k(>1) such that 2^(k-1) = 1 (mod k) and 7^(k-1) = 1 (mod k).