OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..16479 (terms 1..151 from R. J. Mathar)
F. Richman, Primality testing with Fermat's little theorem
FORMULA
a(n) = n-th positive integer k(>1) such that 3^(k-1) = 1 (mod k) and 7^(k-1) = 1 (mod k).
EXAMPLE
a(1)=703 since it is the first number such that 3^(k-1) = 1 (mod k) and 7^(k-1) = 1 (mod k).
MATHEMATICA
Select[Range[420000], !PrimeQ[#]&&PowerMod[3, #-1, #]==1&&PowerMod[7, #-1, #] == 1&] (* Harvey P. Dale, Mar 08 2014 *)
PROG
(PARI) is(n)=!isprime(n)&&Mod(7, n)^(n-1)==1&&Mod(3, n)^(n-1)==1 \\ Charles R Greathouse IV, Apr 12 2012
CROSSREFS
KEYWORD
nonn
AUTHOR
Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), May 05 2003
STATUS
approved