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%I #22 Sep 29 2019 07:06:39
%S 0,0,0,0,0,2,0,3,4,0,6,6,0,2,3,10,0,8,0,9,12,13,17,16,17,8,21,13,22,
%T 14,0,25,22,12,18,22,30,14,17,27,36,29,32,32,25,36,40,37,40,34,18,30,
%U 47,44,40,39,29,46,53,40,0,26,51,55,41,50,62,42,57,44,61
%N a(n) is the base-2 logarithm of the denominator of sigma_{-1}(P(n)), where P(n) = 2^(n-1)*M(n), where M(n) = 2^n - 1 is the n-th Mersenne number.
%C a(n) = 0 if and only if P(n) is multiperfect. In particular, a(n) = 0 if M(n) is prime.
%e a(6) = 2 since P(6) = 2016 and sigma_{-1}(2016) = 13/2^2.
%t M[n_] := 2^n - 1;
%t P[n_] := 2^(n - 1) M[n];
%t A[n_] := Log[2, Denominator[DivisorSigma[-1, P[n]]]];
%o (PARI) a(n) = logint(denominator(sigma(2^(n-1)*(2^n-1),-1)), 2); \\ _Michel Marcus_, Apr 02 2019
%K nonn
%O 1,6
%A _David Terr_, Apr 02 2019
%E More terms from _Felix Fröhlich_, Sep 29 2019
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