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A158521
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Primes which yield primes when "13" is prefixed or appended.
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2
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19, 61, 103, 127, 241, 331, 337, 367, 523, 577, 709, 829, 997, 1009, 1129, 1213, 1231, 1321, 1381, 1489, 1543, 1627, 1861, 2113, 2137, 2287, 2347, 2383, 2689, 2851, 2953, 2971, 3187, 3499, 3559, 3583, 3673, 3967, 4219, 4243, 4327, 4363, 4513, 4591, 4789
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OFFSET
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1,1
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COMMENTS
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It is conjectured that this sequence is infinite.
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REFERENCES
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Richard E. Crandall, Carl Pomerance, Prime Numbers, Springer, 2005.
Wladyslaw Narkiewicz, The development of prime number theory, Springer, 2000.
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LINKS
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FORMULA
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Prime p is a term if the concatenations "13p" and "p13" both yield primes.
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EXAMPLE
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Prime p=3 is not a term: "p13"=313 is prime but "13p"=133 = 7*19.
For p=19, both 1319 and 1913 are prime; this is the first prime that meets the requirements of the definition, so a(1)=19.
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MAPLE
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cat2 := proc(a, b) ndigsb := max(ilog10(b)+1, 1) ; a*10^ndigsb+b ; end: for i from 1 to 800 do p := ithprime(i) ; if isprime(cat2(13, p)) and isprime(cat2(p, 13)) then printf("%d, ", p) ; fi; od: # R. J. Mathar, Apr 02 2009
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MATHEMATICA
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Select[Prime[Range[1000]], AllTrue[{13*10^IntegerLength[#]+#, 100#+13}, PrimeQ]&] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Apr 17 2015 *)
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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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Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Mar 20 2009
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EXTENSIONS
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STATUS
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approved
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