A109992 Numbers n such that the string 99n is prime.

1, 7, 23, 29, 31, 41, 49, 67, 73, 103, 109, 119, 131, 133, 137, 139, 149, 173, 181, 191, 223, 233, 241, 251, 257, 259, 277, 289, 317, 347, 349, 367, 371, 377, 391, 397, 401, 409, 431, 439, 469, 487, 497, 523, 527, 529, 551, 559, 563, 571, 577, 581, 607, 611

Offset

1,2

Links

Vincenzo Librandi, Table of n, a(n) for n = 1..2500

Example

If n=103 then 99n = 99103 (prime).

Maple

a:= proc(n) local k; for k from 1 +`if`(n=1, 0, a(n-1))
      while not isprime(parse(""||99||k)) do od; a(n):=k
    end:
seq(a(n), n=1..100);  # Alois P. Heinz, Jul 30 2014

Mathematica

plQ[n_] := Module[{idn = IntegerDigits[n]}, PrimeQ[FromDigits[PadLeft[idn, Length[idn] + 2, 9]]]]; Select[Range[700], plQ] (* Harvey P. Dale, Jan 18 2011 *)
Select[Range[700], PrimeQ[99 10^IntegerLength[#] + #] &] (* Vincenzo Librandi, Jul 30 2014 *)

Prog

(Magma) [n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [9, 9])) ]; // Vincenzo Librandi, Jul 30 2014

Crossrefs

Sequence in context: A031450 A031116 A044068 * A070330 A095314 A038948

Adjacent sequences: A109989 A109990 A109991 * A109993 A109994 A109995

Keyword

nonn,base

Author

Parthasarathy Nambi, Sep 01 2005

Status

approved