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A056698
Numbers k such that 10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
4
1, 15, 41, 83, 95, 341, 551, 669, 989, 1223, 6923, 103703
OFFSET
1,2
COMMENTS
Also numbers k such that (4*10^k-1)/3 is prime.
a(13) > 850000 (from Kamada data). - Robert Price, Oct 19 2014
MATHEMATICA
Do[ If[ PrimeQ[ 10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 30470}]
PROG
(PARI) for(k=1, 1500, if(ispseudoprime(4*(10^k-1)/3+1), print1(k, ", "))) \\ Hugo Pfoertner, Jul 22 2020
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(12) from Kamada data by Robert Price, Oct 19 2014
STATUS
approved