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 A380833 #18 Feb 06 2025 12:28:35 %S A380833 0,1,0,1,0,2,0,1,0,2,0,2,0,2,1,1,0,2,0,2,0,2,0,1,0,2,0,2,0,4,0,1,0,2, %T A380833 1,2,0,2,0,1,0,3,0,1,0,2,0,2,0,2,0,2,0,2,0,2,0,2,0,1,0,2,0,1,0,4,0,2, %U A380833 0,2,0,2,0,2,0,1,0,3,0,2,0,2,0,3,0,2,0,1,0,3,1,1,0,2,0,1,0,2,0,2 %N A380833 a(n) is the number of divisors d of n satisfying (-d)^n mod n = d. %t A380833 a[n_] := DivisorSum[n, 1 &, PowerMod[-#, n, n] == # &]; Array[a, 100] (* _Amiram Eldar_, Feb 06 2025 *) %o A380833 (Magma) [#[d: d in Divisors(n) | (-d)^n mod n eq d]: n in [1..100]]; %Y A380833 Cf. A000005, A371883, A380656. %K A380833 nonn %O A380833 1,6 %A A380833 _Juri-Stepan Gerasimov_, Feb 06 2025