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 A109370 Numbers n such that the string 22n is the decimal expansion of a prime number. 1
 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 (list; graph; refs; listen; history; text; internal format)
 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

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Last modified October 13 16:50 EDT 2019. Contains 327968 sequences. (Running on oeis4.)