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 A056820 #6 Nov 29 2019 04:02:42
%S A056820 1,2,6,20,48,132,240,528,1364,4180,7560,20784,42768,70992,151632,
%T A056820 446208,986880,1688400,4006800,6080256,13843440,42125380,79567488,
%U A056820 152712000,292573440,526051328,797636448,2942476992,7257012896
%N A056820 Euler totient function of Cullen numbers (A002064), n*2^n +1.
%H A056820 Amiram Eldar, <a href="/A056820/b056820.txt">Table of n, a(n) for n = 0..848</a>
%F A056820 a(n) = A000010(A002064(n)). - _Amiram Eldar_, Nov 29 2019
%t A056820 Table[ EulerPhi[ n*2^n + 1 ], {n, 1, 30} ]
%Y A056820 Cf. A000010, A002064.
%K A056820 nonn
%O A056820 0,2
%A A056820 _Robert G. Wilson v_, Aug 31 2000