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 A108848 Palindromic primes in which all internal digits are 9. 5
 191, 797, 19991, 79997, 199999991, 79999999999999999999999999997, 19999999999999999999999999999999999999991, 199999999999999999999999999999999999999999999999999999999999999999999999999999999999991 (list; graph; refs; listen; history; text; internal format)
 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 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). Sequence in context: A347294 A086878 A082445 * A052165 A103733 A217500 Adjacent sequences: A108845 A108846 A108847 * A108849 A108850 A108851 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

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Last modified February 26 20:11 EST 2024. Contains 370352 sequences. (Running on oeis4.)