A108821 Primes such that the outer 2 digits are n and n+1 and all inner digits are 4, where 0 < n < 9.

647, 24443, 84449, 644447, 24444443, 64444444447, 2444444444443, 84444444444449, 2444444444444444444444444444443, 84444444444444444444444444444444444449

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..27

Formula

a(x) = 10^(x+1)*n+floor(10^x*4/9)*10+n+1. Output if a(x) is prime.

Mathematica

Select[Sort[Flatten[Table[FromDigits[Join[{n}, PadRight[{}, m, 4], {n+1}]], {n, 8}, {m, 40}]]], PrimeQ] (* Harvey P. Dale, Mar 17 2019 *)

Prog

(PARI) n10np1(n) = { local(x, y, k); for(x=1, n, for(k=1, 8, y=10^(x+1)*k+floor(10^x*4/9)*10+k+1; if(isprime(y), print1(y", ")) ) ) }

Crossrefs

Sequence in context: A250989 A187434 A046020 * A345702 A035885 A114827

Adjacent sequences: A108818 A108819 A108820 * A108822 A108823 A108824

Keyword

easy,nonn,base

Author

Cino Hilliard, Jul 11 2005

Status

approved