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 A066220 #7 Jul 04 2017 09:47:33
%S A066220 1,1,2,4,6,60,60,120,144,7920,55440,18480,7920,27720,2520,637560,
%T A066220 8288280,480720240,480720240,480720240,480720240,480720240,1442160720,
%U A066220 9854764920,59128589520,59128589520,147821473800,670124014560
%N A066220 Least k > 0 such that t^k = 1 mod (prime(n) - t) for 0 < t < prime(n).
%C A066220 This sequence gives the period length of the base-p representation of HarmonicNumber[p-1]/p^2 (whose numerator is A061002).
%e A066220 a(5) = 6 because 2^6 = 1 mod 9, 3^6 = 1 mod 8, 4^6 = 1 mod 7, 5^6 = 1 mod 6, 6^6 = 1 mod 5, 7^6 = 1 mod 4, 8^6 = 1 mod 3, 9^6 = 1 mod 2 and 6 is the minimal exponent that satisfies this.
%t A066220 a[p_?PrimeQ] := Module[{e = 1}, While[! And @@ Table[Mod[PowerMod[i, e, p - i] - 1, p - i] == 0, {i, p - 1}], e++]; e]; a /@ Prime[Range[10]]
%K A066220 nonn
%O A066220 1,3
%A A066220 Michael Ulm (taga(AT)hades.math.uni-rostock.de), Dec 18 2001