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 A359206 #11 Aug 23 2023 08:42:19
%S A359206 1,5,17,81,257,1345,4097,20737,69633,328705,1048577,5574657,16777217,
%T A359206 83902465,286261249,1359020033,4294967297,22565617665,68719476737,
%U A359206 348967141377,1168499539969,5497562333185,17592186044417,93531519582209,282574488338433
%N A359206 a(n) = Sum_{d|n} 4^(n-d).
%F A359206 G.f.: Sum_{k>=1} 4^(k-1) * x^k/(1 - 4^(k-1) * x^k).
%F A359206 G.f.: Sum_{k>=1} x^k/(1 - (4 * x)^k).
%t A359206 a[n_] := DivisorSum[n, 4^(n-#) &]; Array[a, 25] (* _Amiram Eldar_, Aug 23 2023 *)
%o A359206 (PARI) a(n) = sumdiv(n, d, 4^(n-d));
%o A359206 (PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, 4^(k-1)*x^k/(1-4^(k-1)*x^k)))
%o A359206 (PARI) my(N=30, x='x+O('x^N)); Vec(sum(k=1, N, x^k/(1-(4*x)^k)))
%Y A359206 Cf. A074854, A112329, A357051.
%Y A359206 Cf. A342628, A342629, A359204.
%K A359206 nonn
%O A359206 1,2
%A A359206 _Seiichi Manyama_, Dec 20 2022