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%I #23 Sep 08 2022 08:46:09
%S 1,7,12,15,14,28,24,60,39,42,63,60,32,90,72,63,124,124,60,120,96,84,
%T 168,124,80,195,120,195,186,168,96,210,224,126,252,195,114,224,252,
%U 186,133,224,280,360,234,248,255,360,171,392,216,210,508,280,216,300
%N a(n) = sigma(sigma(2n-1)).
%C See A247821 - numbers n such that sigma(sigma(2n-1)) is a prime p.
%H Indranil Ghosh, <a href="/A247954/b247954.txt">Table of n, a(n) for n = 1..10000</a>
%F a(n) = A000203(A000203(2n-1)) = A000203(A008438(n-1)) = A051027(2n-1).
%e For n=2; a(2) = sigma(sigma(2*2-1)) = sigma(sigma(3)) = sigma(4) = 7.
%p with(numtheory): A247954:=n->sigma(sigma(2*n-1)): seq(A247954(n), n=1..50); # _Wesley Ivan Hurt_, Oct 01 2014
%t Table[DivisorSigma[1, DivisorSigma[1, 2 n - 1]], {n, 50}] (* _Wesley Ivan Hurt_, Oct 01 2014 *)
%o (Magma) [SumOfDivisors(SumOfDivisors(2*n-1)): n in [1..1000]]
%o (PARI) vector(100,n,sigma(sigma(2*n-1))) \\ _Derek Orr_, Sep 29 2014
%Y Cf. A000203, A051027, A008438, A247820.
%K nonn,easy
%O 1,2
%A _Jaroslav Krizek_, Sep 28 2014
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