A108842 Palindromic primes in which all internal digits are 5.

151, 353, 757, 15551, 75557, 355555553, 75555555557, 155555555555555555551, 755555555555555555557, 75555555555555555555557, 155555555555555555555555555555551, 75555555555555555555555555555555555555555555555555555555557

Offset: 1

Author

Cino Hilliard, Jul 11 2005

Keywords

• easy
• nonn
• base

Comments

The next term -- a(13) -- has 75 digits. - Harvey P. Dale, May 18 2015

a(25) has 1975 digits. - Michael S. Branicky, Jan 27 2023

Links

• Michael S. Branicky, Table of n, a(n) for n = 1..24

Crossrefs

Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), this sequence (d=5), A108843 (d=6), A108844 (d=7), A108847 (d=8).

Programs

Mathematica

Select[Sort[Flatten[Table[FromDigits[Join[{n},PadRight[{},i,5],{n}]],{n,{1,3,7,9}},{i,80}]]],PrimeQ] (* Harvey P. Dale, May 18 2015 *)

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 + "5"*i + f)))
print(list(islice(agen(), 10))) # Michael S. Branicky, Jan 27 2023

Extensions

Name changed by Arkadiusz Wesolowski, Sep 07 2011

More terms from Harvey P. Dale, May 18 2015