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 A056742 #8 Jun 09 2024 09:05:19
%S A056742 1,3,4,15,18,63,64,216,300,968,864,4095,5292,13500,16384,65535,69984,
%T A056742 262143,240000,889056,1320352,4105040,3317760,16200000,22358700,
%U A056742 56733696,66382848,266913216,267300000,1073741823,1073741824
%N A056742 a(n) = phi(2^n - 1)/2.
%H A056742 Amiram Eldar, <a href="/A056742/b056742.txt">Table of n, a(n) for n = 2..1206</a>
%F A056742 a(n) = A000010(A000225(n))/2 = A053287(n)/2. - _Amiram Eldar_, Jun 09 2024
%t A056742 Table[EulerPhi[(2^n - 1)]/2, {n, 2, 40}]
%o A056742 (PARI) a(n) = eulerphi(2^n - 1)/2; \\ _Amiram Eldar_, Jun 09 2024
%Y A056742 Cf. A000010, A000225, A011260, A053287.
%K A056742 nonn
%O A056742 2,2
%A A056742 _Robert G. Wilson v_, Aug 14 2000