A286916 Positive numbers k such that (10^(k+2)*331 - 133)/9 is prime.

7, 13, 23, 409, 1147, 2597, 4561

Offset

1,1

Comments

Or '36'||...'7'...||'63' in decimal form is prime ('7' concatenated k times to which the prefix '36' and the suffix '63' are concatenated once).

a(1)..a(4) and a(7) themselves are primes. a(8) > 50000 (if it exists).

Links

Table of n, a(n) for n=1..7.

Example

7 is a term as 36777777763 is prime (as a string, it consists of '7' concatenated 7 times to which the prefix '36' and the suffix '63' are concatenated once).

Mathematica

ParallelMap[ If[ PrimeQ[ (10^(#+2)*331-133)/9], #, Nothing]&, Range[5000]]

Prog

(PARI) is(n)=ispseudoprime((10^(n+2)*331-133)/9) \\ Charles R Greathouse IV, Jun 13 2017

Crossrefs

Sequence in context: A316118 A265766 A228727 * A032690 A188557 A356966

Adjacent sequences: A286913 A286914 A286915 * A286917 A286918 A286919

Keyword

nonn,hard,more,base

Author

Mikk Heidemaa, May 17 2017

Status

approved