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 A070278 Smallest prime > 2n+1 beginning and ending with 2n+1, or 0 if no such prime exists. 2
 11, 313, 0, 727, 919, 11311, 13313, 0, 17117, 19219, 21121, 23623, 0, 27127, 29129, 31231, 33533, 0, 37337, 39139, 41141, 43543, 0, 47147, 49549, 51151, 53353, 0, 57457, 59159, 61261, 63463, 0, 67567, 692269, 71171, 73673, 0, 77377, 79279 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,1 COMMENTS No such prime exists if n is even or divisible by 5. It is not until n = 69 that it necessary to insert more than one digit. The next few examples are 99, 101, 103, 123, 157, 181 & 199. LINKS MATHEMATICA f[n_] := Block[{a = IntegerDigits[n]}, If[ Mod[n, 5] == 0, Return[0], If[ p = FromDigits[ Join[a, a]]; PrimeQ[p], Return[p], k = 0; While[p = FromDigits[ Join[a, IntegerDigits[k], a]]; !PrimeQ[p], k++ ]]; p]]; Table[ f[n], {n, 1, 89, 2}] CROSSREFS Sequence in context: A185071 A060495 A251589 * A090272 A090271 A267753 Adjacent sequences:  A070275 A070276 A070277 * A070279 A070280 A070281 KEYWORD base,nonn AUTHOR Amarnath Murthy, May 07 2002 EXTENSIONS Edited by N. J. A. Sloane, Vladeta Jovovic and Robert G. Wilson v, May 09 2002 STATUS approved

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Last modified May 13 00:11 EDT 2021. Contains 343829 sequences. (Running on oeis4.)