A353680 - OEIS

%I #11 Nov 01 2025 15:58:10

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

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

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

%N a(n) = 1 if n is odd and phi(sigma(n)) is equal to phi(n), otherwise 0.

%H Antti Karttunen, <a href="/A353680/b353680.txt">Table of n, a(n) for n = 1..100000</a>

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

%H <a href="/index/Si#SIGMAN">Index entries for sequences related to sigma(n)</a>

%F a(n) = A000035(n) * A353637(n).

%t a[n_] := Boole[OddQ[n] && EulerPhi[n] == EulerPhi[DivisorSigma[1, n]]]; Array[a, 100] (* _Amiram Eldar_, May 06 2022 *)

%t Table[If[OddQ[n]&&EulerPhi[DivisorSigma[1,n]]==EulerPhi[n],1,0],{n,150}] (* _Harvey P. Dale_, Nov 01 2025 *)

%o (PARI) A353680(n) = ((n%2) && (eulerphi(sigma(n))==eulerphi(n)));

%Y Characteristic function of A353679.

%Y Cf. A000010, A000035, A000203, A062401, A353637.

%K nonn

%O 1

%A _Antti Karttunen_, May 06 2022