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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, 377777777777777777, 2777777777777777777, 97777777777777777777, 2477777777777777777777
OFFSET
1,1
LINKS
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
KEYWORD
nonn,base
AUTHOR
Robert G. Wilson v, Nov 28 2001
STATUS
approved