A083733 - OEIS

%I #19 Jun 29 2019 08:33:52

%S 561,1105,2465,3277,8321,10585,18721,29341,46657,62745,75361,104653,

%T 115921,162401,219781,226801,252601,278545,294409,314821,334153,

%U 340561,399001,410041,449065,488881,512461,530881,534061,552721,574561,587861

%N Pseudoprimes to bases 2 and 7.

%H Amiram Eldar, <a href="/A083733/b083733.txt">Table of n, a(n) for n = 1..15498</a> (terms 1..138 from R. J. Mathar)

%H F. Richman, <a href="http://math.fau.edu/Richman/carm.htm">Primality testing with Fermat's little theorem</a>

%H <a href="/index/Ps#pseudoprimes">Index entries for sequences related to pseudoprimes</a>

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

%e 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).

%o (PARI) is(n)=!isprime(n)&&Mod(2,n)^(n-1)==1&&Mod(7,n)^(n-1)==1 \\ _Charles R Greathouse IV_, Apr 12 2012

%Y Intersection of A005938 and A001567. - _R. J. Mathar_, Apr 05 2011

%K nonn

%O 1,1

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