OFFSET
1,1
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..15806 (terms 1..147 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 5^(k-1) = 1 (mod k).
EXAMPLE
a(1)=1541 since it is the first nonprime number such that 3^(k-1) = 1 (mod k) and 5^(k-1) = 1 (mod k). - clarified by Harvey P. Dale, Jan 29 2013
MATHEMATICA
Select[Range[420000], !PrimeQ[#]&&PowerMod[3, #-1, #]==PowerMod[5, #-1, #]==1&] (* Harvey P. Dale, Jan 29 2013 *)
CROSSREFS
KEYWORD
easy,nonn
AUTHOR
Serhat Sevki Dincer (sevki(AT)ug.bilkent.edu.tr), May 05 2003
STATUS
approved