A109370 Numbers n such that the string 22n is the decimal expansion of a prime number.

3, 7, 9, 13, 21, 37, 39, 43, 51, 67, 69, 73, 81, 87, 93, 97, 109, 111, 123, 129, 133, 147, 153, 157, 159, 171, 189, 193, 229, 247, 259, 271, 273, 277, 279, 283, 291, 303, 307, 343, 349, 367, 369, 381, 391, 397, 409, 433, 441, 447, 453, 469, 481, 483, 501, 511

Offset

1,1

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..2000

Example

3 is in the sequence because 223 is prime.
67 is in the sequence because 2267 is prime.
111 is in the sequence because 22111 is prime.

Maple

a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))
      while not isprime(parse(""||22||k)) do od; a(n):=k
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Jul 30 2014

Mathematica

Select[Range[700], PrimeQ[22 10^IntegerLength[#] + #] &] (* Vincenzo Librandi, Jul 30 2014 *)

Prog

(Magma) [ n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [2, 2])) ]; // Klaus Brockhaus, Feb 03 2011

Crossrefs

Sequence in context: A118643 A043772 A176553 * A018663 A110575 A325556

Adjacent sequences: A109367 A109368 A109369 * A109371 A109372 A109373

Keyword

nonn,base

Author

Parthasarathy Nambi, Aug 24 2005

Status

approved