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 A065581 Smallest prime ending in exactly n 7's. 6
 7, 277, 1777, 47777, 2477777, 16777777, 137777777, 577777777, 1777777777, 67777777777, 377777777777, 16777777777777, 17777777777777, 577777777777777, 2777777777777777, 157777777777777777 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Harry J. Smith, Table of n, a(n) for n = 1..100 MATHEMATICA Do[a = Table[7, {n} ]; k = 0; While[ b = FromDigits[ Join[ IntegerDigits[k], a]]; Mod[k, 10] == 7 || !PrimeQ[b], k++ ]; Print[b], {n, 1, 17} ] k7[n_]:=Module[{c=FromDigits[PadRight[{}, n, 7]], k=0}, While[!PrimeQ[ k*10^n+ c], k++]; k*10^n+c]; Array[k7, 20] (* Harvey P. Dale, Jan 29 2013 *) PROG (PARI) { for (n=1, 100, t=10^n; b=7*(t - 1) / 9; d=0; while(!isprime(b + t*d), d++; if (d%10==7, d++)); write("b065581.txt", n, " ", b + t*d) ) } \\ Harry J. Smith, Oct 23 2009 CROSSREFS Cf. A037067, A065590, A065821, A065580, A065582. Sequence in context: A066413 A222942 A289634 * A062646 A009503 A209889 Adjacent sequences:  A065578 A065579 A065580 * A065582 A065583 A065584 KEYWORD nonn,base AUTHOR Robert G. Wilson v, Nov 28 2001 STATUS approved

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Last modified January 21 16:22 EST 2020. Contains 331114 sequences. (Running on oeis4.)