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 A027742 #18 Nov 30 2024 06:27:15
%S A027742 1,2,6,16,60,144,756,2048,7776,24000,120032,276480,1719900,4741632,
%T A027742 17820000,67108864,336849900,725594112,4822382628,11842560000,
%U A027742 57802864896,200778006528,998132265920,2283043553280,13122000000000,44980696051200,178118842613760
%N A027742 a(n) = phi(4^n-1)/(2*n).
%H A027742 Amiram Eldar, <a href="/A027742/b027742.txt">Table of n, a(n) for n = 1..1122</a>
%F A027742 a(n) = A295501(n)/(2*n) = A027695(n)/2. - _Amiram Eldar_, Nov 30 2024
%t A027742 Table[EulerPhi[4^n-1]/(2n),{n,30}] (* _Harvey P. Dale_, May 01 2013 *)
%o A027742 (PARI) a(n) = eulerphi(4^n-1)/(2*n) \\ _Felix Fröhlich_, Dec 02 2019
%Y A027742 Cf. A000010, A027695, A295501.
%K A027742 nonn
%O A027742 1,2
%A A027742 _N. J. A. Sloane_
%E A027742 Offset corrected by _Sean A. Irvine_, Dec 02 2019