OFFSET
1,2
COMMENTS
Square terms are 1, 3^2, 3^2*1093^2, 3^2*3511^2 and there are no other squares k^2 with k < 3*10^9. Note that 1093 and 3511 are Wieferich primes (A001220). - Giovanni Resta, Sep 19 2017
EXAMPLE
19313 = 7*31*89 is a term because d(19313) = 2^3 and 7*31*89 divides 2^(7*31*89) - 2^8.
MATHEMATICA
Select[Range[1, 10^7, 2], PowerMod[2, DivisorSigma[0, #], #] == PowerMod[2, #, #] &] (* Michael De Vlieger, Sep 19 2017 *)
PROG
(PARI) isok(n) = n%2==1 && Mod(2, n)^n==2^numdiv(n);
CROSSREFS
KEYWORD
nonn
AUTHOR
Altug Alkan, Sep 19 2017
STATUS
approved