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 A249375 Prime numbers Q such that the concatenation Q,2,Q is prime. 3
 7, 19, 31, 61, 79, 193, 283, 367, 373, 421, 499, 547, 619, 733, 751, 883, 997, 1021, 1033, 1039, 1069, 1153, 1171, 1279, 1399, 1483, 1543, 1567, 1753, 1831, 1879, 1951, 1999, 2083, 2161, 2179, 2251, 2281, 2287, 2503, 2671, 2707, 2713 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Pierre CAMI, Table of n, a(n) for n = 1..10000 EXAMPLE 323 is composite, 525 is composite, 727 is prime so a(1)=7. MATHEMATICA cq2Q[n_]:=Module[{idn=IntegerDigits[n]}, PrimeQ[FromDigits[Join[idn, {2}, idn]]]]; Select[Prime[Range[400]], cq2Q] (* Harvey P. Dale, Apr 17 2019 *) PROG (PFGW & SCRIPT), pre10.txt file with the first 10000000 prime numbers. SCRIPT DIM i, 0 DIM j DIM k DIM n, 1 OPENFILEOUT myf, a(n).txt OPENFILEIN maf, pre10.txt GETNEXT j, maf LABEL loop1 GETNEXT j, maf IF j>10^n THEN SET n, n+1 SET k, j*10^(n+1)+2*10^n+j PRP k IF ISPRP THEN GOTO w GOTO loop1 LABEL w SET i, i+1 WRITE myf, j IF i>9999 THEN END GOTO loop1 (PARI) lista(nn) = {forprime(p=1, nn, if (isprime(eval(concat(concat(Str(p), 2), Str(p)))), print1(p, ", ")); ); } \\ Michel Marcus, Oct 27 2014 (MAGMA) [p: p in PrimesUpTo(3000) | IsPrime(Seqint(Intseq(p) cat [2] cat Intseq(p)))]; // Vincenzo Librandi, Oct 27 2014 CROSSREFS Cf. similar sequences listed in A249374. Sequence in context: A298019 A169605 A216532 * A212492 A234310 A141338 Adjacent sequences:  A249372 A249373 A249374 * A249376 A249377 A249378 KEYWORD nonn,base AUTHOR Pierre CAMI, Oct 27 2014 STATUS approved

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Last modified June 15 12:29 EDT 2021. Contains 345048 sequences. (Running on oeis4.)