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A108844 Palindromic primes in which all internal digits are 7. 9
373, 1777771, 377777777777773, 1777777777777777777777777777777777777777777777771, 3777777777777777777777777777777777777777777777777777773, 377777777777777777777777777777777777777777777777777777777777777777773 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
COMMENTS
a(13) has 1003 digits. - Michael S. Branicky, Jan 27 2023
LINKS
Michael S. Branicky, Table of n, a(n) for n = 1..12
MATHEMATICA
Union[Flatten[Table[Select[Table[FromDigits[Join[{i}, PadRight[{}, n, 7], {i}]], {n, 100}], PrimeQ], {i, 1, 9, 2}]]] (* Harvey P. Dale, Nov 22 2012 *)
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 "1379" if isprime(t:=int(f + "7"*i + f)))
print(list(islice(agen(), 12))) # 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), A108847 (d=8).
Sequence in context: A226850 A208834 A171216 * A025336 A025328 A045284
KEYWORD
easy,nonn,base
AUTHOR
Cino Hilliard, Jul 11 2005
EXTENSIONS
Name changed by Arkadiusz Wesolowski, Sep 07 2011
One more term (a(6)) added by Harvey P. Dale, Nov 22 2012
STATUS
approved

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Last modified April 23 08:10 EDT 2024. Contains 371905 sequences. (Running on oeis4.)