A257531 - OEIS

The OEIS is supported by the many generous donors to the OEIS Foundation.

A257531

If 2^(n-1) mod n = 1, then 1 else 0.

0, 0, 1, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 1, 0, 1, 0

(list; graph; refs; listen; history; text; internal format)

OFFSET

COMMENTS

This first differs from A010051 (1 if n is prime, else 0) at the second term. The next position differing from A010051 is at the 341st term, and further divergences appear for odd pseudoprimes to base 2 (A001567).

LINKS

Antti Karttunen, Table of n, a(n) for n = 1..68101

Index entries for characteristic functions

EXAMPLE

a(341) = 1 as 2^340 mod 341 is 1.

MAPLE

[0, seq(`if`(2 &^ (n-1) mod n = 1, 1, 0), n = 2..104)]; # Peter Luschny, Sep 19 2017

MATHEMATICA

f[n_] := If[PowerMod[2, n - 1, n] == 1, 1, 0]; Array[f, 105] (* Robert G. Wilson v, Apr 28 2015 *)

PROG

(PARI) A257531(n) = if(n==1, 0, if(Mod(2, n)^(n-1)==1, 1, 0)); \\ Antti Karttunen, Michel Marcus & Altug Alkan, Sep 13-20 2017

CROSSREFS