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A109370
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Numbers n such that the string 22n is the decimal expansion of a prime number.
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1
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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
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OFFSET
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1,1
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LINKS
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EXAMPLE
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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.
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MAPLE
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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:
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MATHEMATICA
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Select[Range[700], PrimeQ[22 10^IntegerLength[#] + #] &] (* Vincenzo Librandi, Jul 30 2014 *)
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PROG
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(Magma) [ n: n in [1..600] | IsPrime(Seqint(Intseq(n) cat [2, 2])) ]; // Klaus Brockhaus, Feb 03 2011
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CROSSREFS
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KEYWORD
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nonn,base
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AUTHOR
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STATUS
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approved
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