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 A070806 #15 Jun 13 2024 23:50:28
%S A070806 3,5,7,13,17,29,97,113,193,257,449,509,769,7937,12289,65537,114689,
%T A070806 520193,786433,7340033,8388593,33292289,33550337,469762049,2130706433,
%U A070806 3221225473,8588886017,137438691329,206158430209
%N A070806 Primes p such that cototient(totient(p)) = A070556(p) is a power of 2.
%e A070806 Powers of 2 observable in A070556[this sequence] = {1, 2, 4, 8, 16, 64, 128, 256, 512, 4096, 8192, 32768, 65536, 262144, 524288, ...}. For F(m), Fermat prime:phi[F(m)]=2^m, cototient[2^m]=2^(m-1); if n=113: phi[113]=112, cototient[112]=112-48=64, so 113 is in this sequence.
%t A070806 Do[s= EulerPhi[n]-EulerPhi[EulerPhi[n]]; If[IntegerQ[Log[2, s]]&&PrimeQ[n], Print[n]], {n, 1, 10000000}]
%o A070806 (PARI) ispow2(n)=n==1<<valuation(n,2);
%o A070806 forprime(p=2,4e9,if(ispow2(p-1-eulerphi(p-1)),print1(p", "))) \\ _Charles R Greathouse IV_, May 17 2011
%Y A070806 Cf. A070556, A051953, A054571, A070807, A070809-A070811.
%K A070806 nonn,more
%O A070806 1,1
%A A070806 _Labos Elemer_, May 08 2002
%E A070806 a(20)-a(27) from _Donovan Johnson_, Feb 06 2010
%E A070806 a(28)-a(29) from _Charles R Greathouse IV_, May 17 2011