A355938 - OEIS

%I #10 Jul 21 2022 16:16:25

%S 1,1,1,1,1,0,0,1,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,

%T 0,0,0,0,0,0,1,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,1,0,0,0,0,1,0,0,0,0,

%U 0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,1

%N a(n) = 1 if sigma(n^2) is a noncomposite, otherwise 0.

%H Antti Karttunen, <a href="/A355938/b355938.txt">Table of n, a(n) for n = 1..100000</a>

%H <a href="/index/Ch#char_fns">Index entries for characteristic functions</a>

%F a(n) = A080339(A000203(n^2)) = A080339(A065764(n)).

%F For all n >= 1, a(n) <= A010055(n).

%t a[n_] := If[!CompositeQ[DivisorSigma[1, n^2]], 1, 0]; Array[a, 100] (* _Amiram Eldar_, Jul 21 2022 *)

%o (PARI) A355938(n) = ((1==n)||isprime(sigma(n^2)));

%Y Characteristic function of {1} UNION A055638.

%Y Cf. A000203, A010055, A065764, A080339.

%Y Cf. also A355937.

%K nonn

%O 1

%A _Antti Karttunen_, Jul 21 2022