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 A258454 #29 May 31 2025 19:54:15 %S A258454 3,4,5,8,17,32,257,512,527,992,1952,2522,5252,6512,7412,10376,23432, %T A258454 23717,26732,27302,35114,36632,37442,45872,47027,49022,51092,65537, %U A258454 78899,84242,92432,98432,98672,114767,115292,131072,227222,231167,240977,328352,369272 %N A258454 Numbers n such that phi(n) = 2*phi(n-2). %C A258454 Conjecture: a prime p is in the sequence iff p is a Fermat prime (A019434). %C A258454 This is not correct: the first non-Fermat prime term is 83623937 = 2^18*11*29 + 1. - _Joerg Arndt_, Oct 11 2015 %e A258454 phi(32) = 16 = 2*phi(30) = 2*8, so 32 is in the sequence. %t A258454 Select[Range@ 400000, EulerPhi@ # == 2 EulerPhi[# - 2] &] (* _Michael De Vlieger_, Sep 25 2015 *) %o A258454 (Magma) [n: n in [3..10000000] | EulerPhi(n) eq 2*EulerPhi(n-2)]; %o A258454 (PARI) for(n=1, 1e6, if(eulerphi(n) == 2*eulerphi(n-2), print1(n", "))); \\ _Altug Alkan_, Sep 26 2015 %Y A258454 Cf. A171271, A171262. %K A258454 nonn %O A258454 1,1 %A A258454 _Jaroslav Krizek_, Sep 24 2015