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 A074057 #17 Oct 21 2022 00:00:47
%S A074057 0,-1,0,-1,2,-3,4,-1,2,-3,8,-5,10,-3,0,-1,14,-7,16,-5,2,-3,20,-9,14,
%T A074057 -3,8,-5,26,-15,28,-1,6,-3,12,-13,34,-3,8,-9,38,-19,40,-5,2,-3,44,-17,
%U A074057 34,-11,12,-5,50,-19,24,-9,14,-3,56,-29,58,-3,8,-1,30,-27,64,-5,18,-23,68,-25,70,-3,4,-5,42,-31,76,-17,26,-3,80
%N A074057 a(n) = 2*phi(n-2)-(n-1).
%C A074057 Conjecture : a(n)=0 if and only if n is a Fermat prime (A019434).
%C A074057 The conjecture appears to be false, a(83623937)=0 and 83623937 is not a Fermat number (A000215). See A232720 for sequence of n such that a(n)=0. - _Michel Marcus_, Nov 28 2013
%F A074057 a(n)=-1 if n is of the form 2^x+2;
%F A074057 a(n)=-3 if n is in a subsequence b(k) = 8, 12, 16, 24, 28, 36, 40, 48, 60, 64, 76, 84, 88, 96..( b(k) seems to be asymptotic to c*n*log(n) with c=2.28..).
%t A074057 Table[2*EulerPhi[n-1]-n,{n,2,90}] (* _Harvey P. Dale_, May 22 2020 *)
%o A074057 (PARI) a(n) = 2*eulerphi(n-2) - (n-1); \\ _Michel Marcus_, Nov 28 2013
%Y A074057 Cf. A000010.
%K A074057 easy,sign
%O A074057 3,5
%A A074057 _Benoit Cloitre_, Sep 15 2002