A158972 - OEIS

A158972

a(n) is the smallest number m such that m n's - 1 is prime and zero if there is no such m.

0, 4, 1, 1, 0, 1, 0, 1, 0, 2, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 2, 0, 1, 0, 0, 0, 3, 0, 1, 0, 1, 0, 2, 0, 0, 0, 1, 0, 3270, 0, 1, 0, 1, 0, 0, 0, 1, 0, 3, 0, 957, 0, 1, 0, 0, 0, 2, 0, 1, 0, 1, 0, 5853, 0, 0, 0, 1, 0, 1, 0, 1, 0, 1, 0, 0, 0, 2, 0, 1, 0, 783, 0, 1, 0

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OFFSET

1,2

COMMENTS

I. If m=1 or m is an odd number greater than 3 then a(m)=0. II. If m is greater

than 6 and mod(m,10)=6 then a(m)=0. III. If n-1 is prime the a(n)=1.

a(64) is greater than 4000.

LINKS

Table of n, a(n) for n=1..85.

EXAMPLE

2-1, 22-1 & 222-1 aren't prime and 2222-1 is prime so a(2)=4. There is no m

such that m 16's -1 is prime because 5 divides all such numbers, so a(16)=0.