This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A307961 #10 Jul 25 2025 09:00:13
%S A307961 0,1,9,94,940,9359,93611,936098,9361056,93610484,936104756,9361047434
%N A307961 The number of coreful perfect numbers (A307958) below 10^n.
%F A307961 a(n) ~ c * 10^n, were c = 0.009361... is the asymptotic density of the coreful perfect numbers (A307960).
%t A307961 f[p_,e_] := (p^(e+1)-1)/(p-1)-1; csigma[1]=1; csigma[n_] := Times @@ (f @@@ FactorInteger[n]); cpQ[n_] := csigma[n] == 2*n; s={}; c=0; p=100; Do[If[k==p, AppendTo[s, c]; p*=10]; If[cpQ[k], c++], {k, 1, 1000001}]; s
%Y A307961 Cf. A307822, A307958, A307960.
%K A307961 nonn,more
%O A307961 1,3
%A A307961 _Amiram Eldar_, May 08 2019
%E A307961 a(11)-a(12) from _Amiram Eldar_, Jul 25 2025