A065581 Smallest prime ending in exactly n 7's.

7, 277, 1777, 47777, 2477777, 16777777, 137777777, 577777777, 1777777777, 67777777777, 377777777777, 16777777777777, 17777777777777, 577777777777777, 2777777777777777, 157777777777777777, 377777777777777777, 2777777777777777777, 97777777777777777777, 2477777777777777777777

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[Nand[PrimeQ[k*10^n + c], Mod[k, 10] != 7], k++]; k*10^n+c]; Array[k7, 20] (* Harvey P. Dale, Jan 29 2013 *)

Prog

(PARI) a(n)={ my(t=10^n, b=7*(t-1)/9, d=0); while (!isprime(b + t*d), d++; if(d%10==7, d++)); b + t*d } \\ Harry J. Smith, Oct 23 2009

Crossrefs

Cf. A037067, A065590, A065821, A065580, A065582.

Sequence in context: A332127 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