A108818 - OEIS

%I #10 Jun 14 2020 10:21:30

%S 617,2113,81119,21111113,81111119,8111111119,21111111111113,

%T 81111111111111111119,81111111111111111111119,

%U 2111111111111111111111113,2111111111111111111111111111113,611111111111111111111111111111117

%N Primes such that the outer 2 digits are n and n+1 and all inner digits are 1, where 0 < n < 9.

%C The first digit of each term must be 2, 6, or 8. - _Harvey P. Dale_, Jun 14 2020

%H Harvey P. Dale, <a href="/A108818/b108818.txt">Table of n, a(n) for n = 1..30</a>

%F a(x) = 10^(x+1)*n+floor(10^x/9)*10+n+1. Output if a(x) is prime.

%t Select[Flatten[Table[FromDigits[Join[PadRight[{k},n,1],{k+1}]],{n,2,40},{k,{2,6,8}}]],PrimeQ] (* _Harvey P. Dale_, Jun 14 2020 *)

%o (PARI) n10np1(n) = { local(x,y,k); for(x=1,n, for(k=1,8, y=10^(x+1)*k+floor(10^x/9)*10+k+1; if(isprime(y),print1(y",")) ) ) }

%K easy,nonn,base

%O 1,1

%A _Cino Hilliard_, Jul 11 2005