A108844 Palindromic primes in which all internal digits are 7.

373, 1777771, 377777777777773, 1777777777777777777777777777777777777777777777771, 3777777777777777777777777777777777777777777777777777773, 377777777777777777777777777777777777777777777777777777777777777777773

Offset: 1

Author

Cino Hilliard, Jul 11 2005

Keywords

• easy
• nonn
• base

Comments

a(13) has 1003 digits. - Michael S. Branicky, Jan 27 2023

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..12

Crossrefs

Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), A108842 (d=5), A108843 (d=6), A108847 (d=8).

Programs

Mathematica

Union[Flatten[Table[Select[Table[FromDigits[Join[{i},PadRight[{},n,7],{i}]],{n,100}],PrimeQ],{i,1,9,2}]]] (* Harvey P. Dale, Nov 22 2012 *)

PARI

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

Python

from sympy import isprime
from itertools import count, islice
def agen(): yield from (t for i in count(1) for f in "1379" if isprime(t:=int(f + "7"*i + f)))
print(list(islice(agen(), 12))) # Michael S. Branicky, Jan 27 2023

Extensions

Name changed by Arkadiusz Wesolowski, Sep 07 2011

One more term (a(6)) added by Harvey P. Dale, Nov 22 2012