A108842 Palindromic primes in which all internal digits are 5.

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

Offset

1,1

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

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 *)

Prog

(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

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).

Sequence in context: A118494 A140022 A211551 * A078858 A217498 A078967

Adjacent sequences: A108839 A108840 A108841 * A108843 A108844 A108845

Keyword

easy,nonn,base

Author

Cino Hilliard, Jul 11 2005

Extensions

Name changed by Arkadiusz Wesolowski, Sep 07 2011

More terms from Harvey P. Dale, May 18 2015

Status

approved