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 A257665 #18 Aug 05 2015 11:52:45 %S A257665 2,12,240,4032,16773120,4294901760,68719214592,1152921503533105152, %T A257665 1329227995784915871750885555673497600, %U A257665 95780971304118053647396688887409314154826126411694080,6582018229284824168619876730229320890292528855852623664389292032 %N A257665 Totient of the n-th perfect number. %F A257665 a(n) = A000010(A000396(n)). %t A257665 Map[EulerPhi, (# (# + 1))/2 & /@ Select[2^Range[100] - 1, PrimeQ]] (* _Michael De Vlieger_, Jul 25 2015, after _Harvey P. Dale_ at A000396 *) %Y A257665 Cf. A000010, A000396, subsequence of A098203. %K A257665 nonn %O A257665 1,1 %A A257665 _Carl Schildkraut_, Jul 12 2015