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 A385541 #6 Jul 07 2025 23:48:07 %S A385541 1,2,1,1,1,2,1,1,1,2,1,2,1,2,1,1,1,2,1,1,1,2,1,1,1,2,1,2,1,4,1,1,1,2, %T A385541 1,2,1,2,1,1,1,2,1,1,1,2,1,2,1,2,1,1,1,2,1,2,1,2,1,1,1,2,1,1,1,3,1,1, %U A385541 1,2,1,1,1,2,1,1,1,2,1,2,1,2,1,3,1,2,1,1,1,3,1,1,1,2,1,1,1,2,1,1 %N A385541 Number of divisors of n such that d^d == (-d)^d == d (mod n). %t A385541 a[n_] := DivisorSum[n, 1 &, PowerMod[#, #, n] == PowerMod[-#, #, n] == Mod[#, n] &]; Array[a, 100] (* _Amiram Eldar_, Jul 03 2025 *) %o A385541 (Magma) [1 + #[d: d in [1..n-1] | n mod d eq 0 and Modexp(d,d,n) eq d and Modexp(-d,d,n) eq d]: n in [1..100]]; %Y A385541 Cf. A000005, A384237, A384854, A385392, A385540. %K A385541 nonn %O A385541 1,2 %A A385541 _Juri-Stepan Gerasimov_, Jul 02 2025