A108846 Palindromic primes in which all internal digits are 2.

727, 929, 72227, 3222223, 9222229, 322222223, 722222227, 9222222222229, 72222222222222222222222222227, 72222222222222222222222222222222222222222222222222222222222222227

Offset: 1

Author

Cino Hilliard, Jul 11 2005

Keywords

• easy
• nonn
• base

Comments

a(14) has 1525 digits. - Michael S. Branicky, Jan 27 2023

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..13

Crossrefs

Similar sequences for digit d: A108845 (d=1), A108841 (d=4), A108842 (d=5), A108843 (d=6), A108844 (d=7), A108847 (d=8), A108848 (d=9).

Programs

Mathematica

Select[Flatten[Table[10 FromDigits[PadRight[{d},n,2]]+d,{d,{1,3,7,9}},{n,2,70}]],PrimeQ]//Sort (* Harvey P. Dale, Feb 05 2023 *)

PARI

n10np1(n,d) = { local(x,y,k); for(x=1,n, for(k=1,9, 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 + "2"*i + f)))
print(list(islice(agen(), 10))) # Michael S. Branicky, Jan 27 2023

Extensions

Name changed by Arkadiusz Wesolowski, Sep 07 2011