OFFSET
1,1
COMMENTS
Numbers (k+1)/2 are listed in A298758.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..3031 (terms below 2^64)
FORMULA
a(n) = 2*A298758(n) - 1.
MATHEMATICA
Select[Cases[Import["https://oeis.org/A001567/b001567.txt", "Table"], {_, _}][[;; , 2]], !PrimeQ[(#+1)/2] && PowerMod[2, (#-1)/2, (#+1)/2] == 1 &] (* Amiram Eldar, Nov 09 2023 *)
PROG
(PARI) isF(n) = {Mod(2, n)^n==2 && !isprime(n) && n>1};
isok(n) = (n%2) && isF(n) && isF((n+1)/2); \\ Michel Marcus, Apr 26 2018
CROSSREFS
KEYWORD
nonn
AUTHOR
Max Alekseyev, Apr 25 2018
STATUS
approved