A066220 - OEIS

A066220

Least k > 0 such that t^k = 1 mod (prime(n) - t) for 0 < t < prime(n).

1, 1, 2, 4, 6, 60, 60, 120, 144, 7920, 55440, 18480, 7920, 27720, 2520, 637560, 8288280, 480720240, 480720240, 480720240, 480720240, 480720240, 1442160720, 9854764920, 59128589520, 59128589520, 147821473800, 670124014560

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OFFSET

1,3

COMMENTS

This sequence gives the period length of the base-p representation of HarmonicNumber[p-1]/p^2 (whose numerator is A061002).

LINKS

Table of n, a(n) for n=1..28.

EXAMPLE

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.

MATHEMATICA

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]]

CROSSREFS

Sequence in context: A333456 A377700 A348244 * A348152 A367854 A009257

Adjacent sequences: A066217 A066218 A066219 * A066221 A066222 A066223

KEYWORD

nonn

AUTHOR

Michael Ulm (taga(AT)hades.math.uni-rostock.de), Dec 18 2001

STATUS

approved