A056051 - OEIS

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A056051

a(n) = (n-2)! - 1 (mod n).

0, 0, 1, 0, -1, 0, -1, -1, -1, 0, -1, 0, -1, -1, -1, 0, -1, 0, -1, -1, -1, 0, -1, -1, -1, -1, -1, 0, -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, -1, 0, -1, 0, -1, -1, -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, -1, -1, -1, 0, -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, -1, 0, -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, -1, 0, -1, -1, -1, -1, -1, 0, -1, -1, -1, -1

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OFFSET

2,1

COMMENTS

(n-2)! - 1 is congruent to 0 iff n is a prime and is congruent to -1 iff n is a composite > 4.

LINKS

Antti Karttunen, Table of n, a(n) for n = 2..10000

MATHEMATICA

Do[m = Mod[(n - 2)! - 1, n]; If[m < 2, Print[m], Print[m - n]], {n, 2, 100}]

PROG

(PARI) A056051(n) = { my(m=(((n-2)!-1)%n)); if(m < 2, m, m-n); }; \\ Antti Karttunen, Sep 14 2017, after Mathematica-code.

CROSSREFS