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 A158521 Primes which yield primes when "13" is prefixed or appended. 1
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Primes in A158232. It is conjectured that this sequence is infinite. REFERENCES Richard E. Crandall, Carl Pomerance, Prime Numbers, Springer, 2005. Wladyslaw Narkiewicz, The development of prime number theory, Springer, 2000. LINKS Harvey P. Dale, Table of n, a(n) for n = 1..1000 FORMULA Prime p is a term if the concatenations "13p" and "p13" both yield primes. EXAMPLE 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. MAPLE 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 MATHEMATICA 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 *) CROSSREFS Cf. A158232, A157772. Sequence in context: A270818 A206426 A211145 * A141897 A031384 A048890 Adjacent sequences:  A158518 A158519 A158520 * A158522 A158523 A158524 KEYWORD nonn,base AUTHOR Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Mar 20 2009 EXTENSIONS 337, 1231, 1321 inserted by R. J. Mathar, Apr 02 2009 STATUS approved

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Last modified April 23 06:08 EDT 2019. Contains 322381 sequences. (Running on oeis4.)