OFFSET
1,1
COMMENTS
The external digits must either be ones or sevens. - Harvey P. Dale, Apr 06 2019
a(15) has 3385 digits. - Michael S. Branicky, Jan 27 2023
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..14
MATHEMATICA
nn=80; With[{o=Table[FromDigits[Join[PadRight[{1}, n, 6], {1}]], {n, 3, nn}], s= Table[ FromDigits[Join[PadRight[{7}, n, 6], {7}]], {n, 3, nn}]}, Select[ Sort[ Join[o, s]], PrimeQ]] (* Harvey P. Dale, May 26 2014 *)
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 "17" if isprime(t:=int(f + "6"*i + f)))
print(list(islice(agen(), 10))) # Michael S. Branicky, Jan 27 2023
CROSSREFS
KEYWORD
easy,nonn,base
AUTHOR
Cino Hilliard, Jul 11 2005
EXTENSIONS
Name changed by Arkadiusz Wesolowski, Sep 07 2011
STATUS
approved