A115558 - OEIS

%I #14 Jan 28 2025 01:46:13

%S 1,7,11,13,19,23,29,31,43,47,53,73,83,97,103,113,127,157,179,197,199,

%T 223,227,233,239,241,251,257,271,281,311,316,317,333,353,389,401,409,

%U 419,421,443,449,461,467,479,491,503,509,549,563,587,593,599,617,641

%N a(n) is the square root of A115557(n).

%H Giovanni Resta, <a href="/A115558/b115558.txt">Table of n, a(n) for n = 1..10000</a>

%F The commutator [sigma, tau] is zero and a(n) is the square root of solutions. Both prime and composite numbers.

%t Select[Range[1000], DivisorSigma[0, DivisorSigma[1, #^2]] == DivisorSigma[1, DivisorSigma[0, #^2]] &] (* _Amiram Eldar_, Jan 28 2025 *)

%o (PARI) isok(k) = numdiv(sigma(k^2)) == sigma(numdiv(k^2)); \\ _Amiram Eldar_, Jan 28 2025

%Y Cf. A000005, A000203, A076360, A076361, A062068, A062069, A115557, A115559, A115560.

%K nonn

%O 1,2

%A _Labos Elemer_, Jan 25 2006