OFFSET
1,1
COMMENTS
Obviously, 1 and 7 are the only possible outer digits for repeating inner digit 9.
Terms a(14), a(15), and a(16) have respectively 1213, 1285, and 1461 digits. - Harvey P. Dale, Dec 11 2019
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..13
MATHEMATICA
Select[Flatten[Table[FromDigits[PadRight[{k}, n, 9]]*10+k, {n, 2, 200}, {k, {1, 7}}]], PrimeQ] (* Harvey P. Dale, Dec 11 2019 *)
PROG
(PARI) n10np9(n, d) = { local(x, y, k); for(x=1, n, for(k=1, 9, y=10^(x+1)*k+(10^x-1)*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 + "9"*i + f)))
print(list(islice(agen(), 13))) # 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
More terms from Harvey P. Dale, Dec 11 2019
STATUS
approved