A193072 - OEIS

%I #21 Oct 10 2022 07:57:04

%S 39,507,2379,6591,13167,29511,148955,1672209,8852259,212370543,

%T 1929229929

%N Odd numbers N for which numerator(sigma(N^2)/N^2) is prime but sigma(N^2) is composite.

%C a(12) > 10^10. - _Lucas A. Brown_, Apr 12 2021

%H Lucas A. Brown, <a href="https://github.com/lucasaugustus/oeis/blob/main/A193072.py">A193072.py</a>

%o (PARI) forstep(N=1, 1e7, 2, isprime(numerator(sigma(N^2)/N^2)) && !isprime(sigma(N^2)) && print1(N", "))

%Y Intersection of A193066 and A193071.

%K nonn,hard,more

%O 1,1

%A _M. F. Hasler_, Jul 15 2011

%E a(8)-a(10) from _Donovan Johnson_, Sep 19 2011

%E a(11) from _Lucas A. Brown_, Apr 12 2021