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

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.

Mathematica

f[n_, m_]:=(v={}; Do[v=Join[v, IntegerDigits[n]], {k, m}]; FromDigits[v]);
a[n_]:=(If[n!=3&&n!=6&&(Mod[n, 10]==6||Mod[n, 2]==1), 0, For[m=1, !PrimeQ[f[n, m]
-1], m++ ]; m]); Do[Print[a[n]], {n, 39}]

Crossrefs

Cf. A158971.

Sequence in context: A299354 A117414 A085639 * A278987 A135302 A364026

Adjacent sequences: A158969 A158970 A158971 * A158973 A158974 A158975

Keyword

nonn,base

Author

Farideh Firoozbakht, Apr 02 2009

Extensions

a(64)-a(85) from Giovanni Resta, Apr 17 2017

Status

approved