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A109992 Numbers n such that the string 99n is prime. 1
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 (list; graph; refs; listen; history; text; internal format)
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

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Last modified June 22 01:32 EDT 2021. Contains 345367 sequences. (Running on oeis4.)