A356451 - OEIS

%I #16 Mar 16 2026 04:37:47

%S 1,2,3,4,5,6,8,9,10,11,12,13,14,15,16,17,18,19,20,22,23,24,25,26,27,

%T 28,29,30,31,32,33,34,36,37,38,40,41,43,44,45,46,47,48,50,51,52,53,54,

%U 55,58,59,60,61,64,65,66,67,68,69,70,71,72,73,74,75,76,79,80,81,82,83

%N Numbers k such that 4*k^2 is in A014567.

%C Numbers k such that k and sigma(4*k^2) are coprime, sigma = A000203.

%H Jianing Song, <a href="/A356451/b356451.txt">Table of n, a(n) for n = 1..7127</a> (all terms <= 10000)

%F a(n) = A356448(n)/2.

%e 12 is a term since 12 and sigma(4*12^2) = 1651 are coprime.

%t q[k_] := CoprimeQ[4*k^2, DivisorSigma[1, 4*k^2]]; Select[Range[100], q] (* _Amiram Eldar_, Mar 16 2026 *)

%o (PARI) isA356450(n) = gcd(n,sigma(4*n^2))==1

%Y Cf. A014567, A000203, A356382, A356448, A356449, A356454 (complement).

%K nonn,easy

%O 1,2

%A _Jianing Song_, Aug 07 2022