A068688 Primes which are a sandwich of numbers made of only one digit between two 7's.

727, 757, 787, 797, 72227, 75557, 76667, 78887, 79997, 7666667, 722222227, 74444444447, 75555555557, 755555555555555555557, 75555555555555555555557, 72222222222222222222222222227, 79999999999999999999999999997, 7444444444444444444444444444447

Offset: 1

Author

Amarnath Murthy, Mar 02 2002

Keywords

• base
• nonn

Comments

The middle digit is never 0, 1, 3, or 7. - Harvey P. Dale, May 05 2018

a(40) has 1213 digits. - Michael S. Branicky, Jan 28 2023

Links

• Harvey P. Dale, Table of n, a(n) for n = 1..39

Crossrefs

Cf. A068685, A068687, A068689.

Programs

Mathematica

Select[Flatten[Table[10FromDigits[PadRight[{7},n,i]]+7,{n,2,100},{i,9}]],PrimeQ] (* Harvey P. Dale, May 05 2018 *)

Python

from sympy import isprime
from itertools import count, islice
def agen(): yield from (t for i in count(1) for m in "0123456789" if isprime(t:=int("7" + m*i + "7")))
print(list(islice(agen(), 30))) # Michael S. Branicky, Jan 28 2023

Extensions

More terms from Sascha Kurz, Mar 17 2002