A158971 a(n) is the smallest number m such that m n's + 1 is prime and zero if there is no such m.

1, 1, 0, 1, 0, 1, 0, 2, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1, 0, 3, 0, 1, 0, 0, 0, 3, 0, 1, 0, 1, 0, 3, 0, 0, 0, 1, 0, 3, 0, 1, 0, 1, 0, 0, 0, 1, 0, 1950, 0, 2, 0, 1, 0, 0, 0, 2, 0, 1, 0, 1, 0, 2, 0, 0, 0, 1, 0, 2, 0, 1, 0, 1, 0, 0, 0, 3, 0, 1, 0, 2, 0, 1, 0, 0, 0, 87, 0, 1, 0, 2, 0, 2, 0, 0, 0, 1, 0, 5, 0, 1, 0, 1, 0, 0

Offset

1,8

Comments

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

than 4 and mod(m,10)=4 then a(n)=0. III. If n+1 is prime the a(n)=1.

Links

Table of n, a(n) for n=1..104.

Example

20+1 & 2020+1 aren't prime but 202020+1 is prime so a(20)=3. If n>4 and
mod(n,10)=4 then there is no number m such that m n's + 1 is prime because
5 divides all such numbers so a(n)=0.

Mathematica

f[n_, m_]:=(v={}; Do[v=Join[v, IntegerDigits[n]], {k, m}]; FromDigits[v]);
a[n_]:=(If[n!=1&&n!=4&&(Mod[n, 10]==4||Mod[n, 2]==1), 0, For[m=1, !PrimeQ[f[n, m]
+1], m++ ]; m]); Do[Print[a[n]], {n, 104}]

Crossrefs

Cf. A158972.

Sequence in context: A374328 A372505 A322338 * A121467 A366073 A362412

Adjacent sequences: A158968 A158969 A158970 * A158972 A158973 A158974

Keyword

nonn

Author

Farideh Firoozbakht, Apr 02 2009

Status

approved