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 A250404 #28 May 05 2024 01:30:05
%S A250404 1,2,3,15,255,65535,4294967295
%N A250404 Numbers k such that the set of all distinct values of phi of all divisors of k equals the set of all proper divisors of k+1 where phi is the Euler totient function (A000010).
%C A250404 Numbers k such that {phi(d) : d|k} = {d : d|(k+1), d<(k+1)} as sets.
%C A250404 Conjecture: last term is 4294967295.
%C A250404 Sequence differs from A203966 because 83623935 is not in this sequence.
%e A250404 2 is a term since {phi(d) : d|2} = {1} = {d; d|2, d<2}.
%e A250404 15 is a term since {phi(d) : d|15} = {1, 2, 4, 8} = {d : d|16, d<16}.
%o A250404 (Magma) [n: n in [1..100000] | Set([EulerPhi(d): d in Divisors(n)]) eq Set([d: d in Divisors(n+1) | d lt n+1 ])]
%o A250404 (PARI) isok(n) = {sphi = []; fordiv(n, d, sphi = Set(concat(sphi, eulerphi(d)))); sphi == setminus(Set(divisors(n+1)), Set(n+1));} \\ _Michel Marcus_, Nov 23 2014
%Y A250404 Subsequence of A203966.
%Y A250404 Cf. A000010, A203966, A250405.
%K A250404 nonn,more,hard
%O A250404 1,2
%A A250404 _Jaroslav Krizek_, Nov 22 2014
%E A250404 Edited and a(7) added by _Max Alekseyev_, May 04 2024