A108844 Palindromic primes in which all internal digits are 7.

373, 1777771, 377777777777773, 1777777777777777777777777777777777777777777777771, 3777777777777777777777777777777777777777777777777777773, 377777777777777777777777777777777777777777777777777777777777777777773

Offset

1,1

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

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 *)

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 + "7"*i + f)))
print(list(islice(agen(), 12))) # Michael S. Branicky, Jan 27 2023

Crossrefs

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

Sequence in context: A226850 A208834 A171216 * A025336 A025328 A045284

Adjacent sequences: A108841 A108842 A108843 * A108845 A108846 A108847

Keyword

easy,nonn,base

Author

Cino Hilliard, Jul 11 2005

Extensions

Name changed by Arkadiusz Wesolowski, Sep 07 2011

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

Status

approved