A268707 - OEIS

%I #17 Mar 21 2016 12:44:59

%S 2,19,199,1999,49999,199999,2999999,19999999,799999999,9199999999,

%T 59999999999,959999999999,9919999999999,59999999999999,

%U 499999999999999,9299999999999999,99919999999999999,994999999999999999,9991999999999999999,29999999999999999999

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

%H Michel Lagneau, Michael De Vlieger and Robert G. Wilson v, <a href="/A268707/b268707.txt">Table of n, a(n) for n = 1..1225</a>

%t 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]

%o (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

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

%K nonn,base

%O 1,1

%A _Michel Lagneau_, _Michael De Vlieger_ and _Robert G. Wilson v_, Feb 11 2016