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

1,1

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

Mathematica

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

Prog

(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

Crossrefs

Cf. A068685, A068687, A068689.

Sequence in context: A084412 A090268 A090267 * A088283 A123096 A108846

Adjacent sequences: A068685 A068686 A068687 * A068689 A068690 A068691

Keyword

base,nonn

Author

Amarnath Murthy, Mar 02 2002

Extensions

More terms from Sascha Kurz, Mar 17 2002

Status

approved