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 A249541 #33 Jun 10 2025 19:07:32 %S A249541 3,4,5,17,257,65537,83623937,4294967297,6992962672132097 %N A249541 Numbers m such that phi(m-2) divides m-1 where phi is Euler's totient function (A000010). %C A249541 The first 5 known Fermat primes from A019434 are in this sequence. %C A249541 Corresponding values of numbers k(m) = (m-1) / phi(m-2): 2, 3, 2, 2, 2, 2, 2, 2, ... %C A249541 Conjecture: 4 is the only number m such that 3*phi(m-2) = m-1. (See comment in A203966.) %F A249541 a(n) = A203966(n+1) + 2. - _Max Alekseyev_, Feb 01 2024 %e A249541 4 is in the sequence because phi(4-2) = 1 divides 4-1 = 3. %o A249541 (Magma) [n: n in [3..10000000] | (n-1) mod EulerPhi(n-2) eq 0]; %Y A249541 Supersequence of A232720 and A254576. %Y A249541 Cf. A000010, A019434, A203966, A232720, A254576. %K A249541 nonn,more %O A249541 1,1 %A A249541 _Jaroslav Krizek_, Feb 25 2015 %E A249541 a(9) confirmed by _Max Alekseyev_, Feb 01 2024