A268707 Smallest n-digit prime having at least n-1 digits equal to 9.

2, 19, 199, 1999, 49999, 199999, 2999999, 19999999, 799999999, 9199999999, 59999999999, 959999999999, 9919999999999, 59999999999999, 499999999999999, 9299999999999999, 99919999999999999, 994999999999999999, 9991999999999999999, 29999999999999999999

Offset: 1

Author

Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Feb 11 2016

Keywords

• nonn
• base

Links

• Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, Table of n, a(n) for n = 1..1225

Crossrefs

Cf. A037071, A241100, A268702 - A268706, A241206, A266148.

Programs

Mathematica

f[n_] := Block[{k = 0, p = {}, r = (10^n - 1), s = Range@ 10 - 10}, While[k < n - 0, AppendTo[p, Select[r + 10^k*s, PrimeQ]]; k++]; p = Min@ Flatten@ p]; Array[f, 20]

PARI

a(n)=my(t=10^n-1,p); forstep(d=n-1,0,-1, forstep(k=8,1,-1, p=t-10^d*k; if(ispseudoprime(p), return(p)))); -1 \\ Charles R Greathouse IV, Mar 21 2016