A342630 - OEIS

%I #7 Mar 18 2021 22:59:20

%S 5,6,7,14,18,24,29,34,39,41,47,53,77,114,119,148,150,159,176,188,189,

%T 190,191,205,215,217,218,241,268,288,312,314,331,334,339,342,346,352,

%U 364,367,387,390,402,418,429,438,439,440,446,449,480,493,494,500,504,510,521,523,546,549,553,561,580

%N Numbers k such that A340180(k) is prime.

%H Robert Israel, <a href="/A342630/b342630.txt">Table of n, a(n) for n = 1..5000</a>

%e a(4) = 14 is a term because A340180(14) = 23 is prime.

%p f:= proc(n) local s,k;

%p   s:= numtheory:-sigma(n);

%p   add(`if`(igcd(n,k)=1, s mod k, 0),k=1..n-1);

%p end proc:

%p select(t -> isprime(f(t)), [$1..1000]);

%Y Cf. A340180.

%K nonn

%O 1,1

%A _J. M. Bergot_ and _Robert Israel_, Mar 17 2021