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 A103967 Numbers n such that the string n1n is the decimal expansion of a prime number. 2
 3, 9, 17, 21, 27, 29, 39, 41, 47, 51, 59, 71, 81, 87, 89, 113, 117, 119, 131, 137, 141, 159, 161, 177, 189, 201, 239, 263, 287, 291, 299, 333, 359, 371, 377, 389, 393, 399, 443, 459, 461, 467, 471, 489, 509, 539, 573, 603, 623, 653, 657, 659, 663, 671, 681 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are odd and not multiples of 5. - Zak Seidov, Oct 29 2014 LINKS Zak Seidov, Table of n, a(n) for n = 1..10000 EXAMPLE n=3 is in the sequence because n1n = 313 is prime. n=41 is in the sequence because n1n = 41141 is prime. n=89 is in the sequence because n1n = 89189 is prime. MATHEMATICA Reap[Do[If[Mod[n, 5]>0&&PrimeQ[ToExpression[ToString[n]<>"1"<>ToString[n]]], Sow[n]], {n, 3, 1000, 2}]][[2, 1]] (* Zak Seidov, Oct 29 2014 *) PROG (MAGMA) [ n: n in [0..800] | IsPrime(Seqint(Intseq(n) cat [1] cat Intseq(n))) ]; // Klaus Brockhaus, Feb 03 2011 CROSSREFS Cf. A249374. Sequence in context: A106676 A050795 A050797 * A032400 A329422 A004621 Adjacent sequences:  A103964 A103965 A103966 * A103968 A103969 A103970 KEYWORD base,nonn AUTHOR Parthasarathy Nambi, Mar 30 2005 STATUS approved

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Last modified May 6 00:41 EDT 2021. Contains 343579 sequences. (Running on oeis4.)