A103011 Numbers n such that 5*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

1, 5, 11, 14, 62, 74, 107, 110, 121, 128, 139, 320, 2956, 11116

Offset

1,2

Comments

Also numbers n such that (47*10^n+61)/9 is prime.

a(15) > 10^5. - Robert Price, Jul 16 2015

Links

Table of n, a(n) for n=1..14.

Makoto Kamada, Prime numbers of the form 522...229.

Index entries for primes involving repunits.

Formula

a(n) = A101576(n) + 1.

Mathematica

Do[ If[ PrimeQ[(46*10^n + 61)/9], Print[n]], {n, 0, 10000}]

Prog

(PARI) repunit(n)=my(r=0, i); for(i=1, n, r=10*r+1); r
f(n)=5*10^n + 2*repunit(n) + 7
main(size)=my(i, r=1, v=vector(size)); for(i=1, size, while(1, if(isprime(f(r)), v[i]=r; r++; break, r++))); v; \\ Anders Hellström, Jul 16 2015

Crossrefs

Cf. A002275, A101576.

Sequence in context: A314001 A196206 A079030 * A137002 A091718 A078002

Adjacent sequences: A103008 A103009 A103010 * A103012 A103013 A103014

Keyword

more,nonn

Author

Robert G. Wilson v, Jan 18 2005

Extensions

Addition of a(14) from Kamada data by Robert Price, Dec 12 2010

Status

approved