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%I #21 Jan 31 2023 08:28:58
%S 191,797,19991,79997,199999991,79999999999999999999999999997,
%T 19999999999999999999999999999999999999991,
%U 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991
%N Palindromic primes in which all internal digits are 9.
%C Obviously, 1 and 7 are the only possible outer digits for repeating inner digit 9.
%C Terms a(14), a(15), and a(16) have respectively 1213, 1285, and 1461 digits. - _Harvey P. Dale_, Dec 11 2019
%H Harvey P. Dale, <a href="/A108848/b108848.txt">Table of n, a(n) for n = 1..13</a>
%t Select[Flatten[Table[FromDigits[PadRight[{k},n,9]]*10+k,{n,2,200},{k,{1,7}}]],PrimeQ] (* _Harvey P. Dale_, Dec 11 2019 *)
%o (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",")) ) ) }
%o (Python)
%o from sympy import isprime
%o from itertools import count, islice
%o def agen(): yield from (t for i in count(1) for f in "17" if isprime(t:=int(f + "9"*i + f)))
%o print(list(islice(agen(), 13))) # _Michael S. Branicky_, Jan 27 2023
%Y Similar sequences for digit d: A108845 (d=1), A108846 (d=2), A108841 (d=4), A108842 (d=5), A108843 (d=6), A108844 (d=7), A108847 (d=8), A108848 (d=9).
%K easy,nonn,base
%O 1,1
%A _Cino Hilliard_, Jul 11 2005
%E Name changed by _Arkadiusz Wesolowski_, Sep 07 2011
%E More terms from _Harvey P. Dale_, Dec 11 2019
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