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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
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
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.)