OFFSET
1,1
PROG
(PARI) f(n) = (eulerphi(2*n+1) + 1 + g(n))/2; \\ A140607
g(n) = sumdiv(2*n+1, d, eulerphi(d)/(t=znorder(Mod(2, d))))*t-t+1; \\ A137576
isok(c) = if (!isprime(c) && (c%2), f((c-1)/2) == c); \\ Michel Marcus, Jan 31 2023
CROSSREFS
KEYWORD
nonn
AUTHOR
Ray Chandler, May 20 2008
EXTENSIONS
More terms from Michel Marcus, Jan 31 2023
STATUS
approved