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 A059910 #15 Jan 26 2025 02:21:34
%S A059910 0,1,4,6,9,4,4,6,20,9,8,2,6,6,12,44,5,6,18,14,12,4,4,2,56,13,20,4,6,2,
%T A059910 40,6,18,12,12,44,63,6,28,4,16,14,8,2,18,12,28,14,70,3,42,12,42,6,24,
%U A059910 8,56,44,60,6,60,2,4,90,21,20,24,2,18,60,88,6,12,2,28,26,6,28,8,14,170
%N A059910 a(n) = |{m : multiplicative order of n mod m = 5}|.
%C A059910 The multiplicative order of a mod m, gcd(a,m) = 1, is the smallest natural number d for which a^d = 1 (mod m).
%F A059910 a(n) = tau(n^5-1)-tau(n-1), where tau(n) = number of divisors of n A000005. Generally, if b(n, r) = |{m : multiplicative order of n mod m = r}| then b(n, r) = Sum_{d|r} mu(d)*tau(n^(r/d)-1), where mu(n) = Moebius function A008683.
%t A059910 a[n_] := Subtract @@ DivisorSigma[0, {n^5-1, n-1}]; a[1] = 0; Array[a, 100] (* _Amiram Eldar_, Jan 25 2025 *)
%o A059910 (PARI) a(n) = if(n == 1, 0, numdiv(n^5-1) - numdiv(n-1)); \\ _Amiram Eldar_, Jan 25 2025
%Y A059910 Cf. A059907-A059909, A059911, A059499, A059885-A059892, A002326, A053446-A053452, A002329, A055205, A048691, A048785.
%K A059910 easy,nonn
%O A059910 1,3
%A A059910 _Vladeta Jovovic_, Feb 08 2001