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 A053634 #20 Dec 26 2017 23:32:53
%S A053634 2,3,4,7,10,18,30,54,94,176,316,591,1096,2058,3856,7301,13798,26244,
%T A053634 49940,95373,182362,349626,671092,1290714,2485534,4793790,9256396,
%U A053634 17896284,34636834,67109898,130150588,252647064,490853416,954440950
%N A053634 a(n) = Sum_{ d divides n } phi(d)*2^(n/d)/(2n).
%C A053634 Offset is 3 because a(2) is 3/2, not an integer. - _Michel Marcus_, Sep 11 2013
%H A053634 G. C. Greubel, <a href="/A053634/b053634.txt">Table of n, a(n) for n = 3..3000</a>
%H A053634 James East, Ron Niles, <a href="https://arxiv.org/abs/1710.11245">Integer polygons of given perimeter</a>, arXiv:1710.11245 [math.CO], 2017.
%F A053634 a(n) = A000031(n)/2.
%t A053634 a[n_] := DivisorSum[n, EulerPhi[#]*2^(n/#)&]/(2n); Table[a[n], {n, 3, 36}] (* _Jean-François Alcover_, Dec 07 2015 *)
%o A053634 (PARI) a(n) = sumdiv (n, d, eulerphi(d)*2^(n/d)/(2*n)); \\ _Michel Marcus_, Sep 11 2013
%Y A053634 Cf. A000016, A000031, A053635, A063776.
%K A053634 nonn
%O A053634 3,1
%A A053634 _N. J. A. Sloane_, Mar 23 2000