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 A347991 #36 Oct 14 2021 08:47:50
%S A347991 1,5,9,69,33,2061,129,16453,4105,131109,2049,134219853,8193,8388741,
%T A347991 8388649,1073758277,131073,274877913101,524289,2199023386725,
%U A347991 2147483785,34359740421,8388609,576460752437659725,1073741857,2199023263749,549755817993,36028797027352773,536870913,2361183241434831128621
%N A347991 a(n) = Sum_{d|n} 2^(sigma(d) - 1).
%H A347991 Seiichi Manyama, <a href="/A347991/b347991.txt">Table of n, a(n) for n = 1..1000</a>
%F A347991 If p is prime, a(p) = 1 + 2^p.
%F A347991 G.f.: Sum_{k>=1} 2^(sigma(k) - 1) * x^k/(1 - x^k).
%t A347991 a[n_] := DivisorSum[n, 2^(DivisorSigma[1, #] - 1) &]; Array[a, 30] (* _Amiram Eldar_, Oct 08 2021 *)
%o A347991 (PARI) a(n) = sumdiv(n, d, 2^(sigma(d)-1));
%o A347991 (PARI) my(N=40, x='x+O('x^N)); Vec(sum(k=1, N, 2^(sigma(k)-1)*x^k/(1-x^k)))
%Y A347991 Cf. A000203, A347405, A348223.
%K A347991 nonn
%O A347991 1,2
%A A347991 _Seiichi Manyama_, Oct 08 2021