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A257036
Numbers k such that 9*R_(k+2) - 8*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
0
1, 2, 8, 13, 45, 46, 118, 463, 668, 1508, 3302, 3970, 4250, 5573, 544905, 833852, 1439761, 1585996
OFFSET
1,2
COMMENTS
Also, numbers k such that 92*10^k - 1 is prime.
Terms a(1)-a(14) from Makoto Kamada.
LINKS
Predrag Kurtovic, 92*10^544905 - 1, The 5000 Largest Known Primes.
Predrag Kurtovic, 92*10^833852 - 1, The 5000 Largest Known Primes.
Predrag Kurtovic, 92*10^1439761 - 1, The 5000 Largest Known Primes.
Predrag Kurtovic, 92*10^1585996 - 1, The 5000 Largest Known Primes.
EXAMPLE
For k=2, 9*R_4 - 8*10^2 = 9999 - 800 = 9199 which is prime.
MATHEMATICA
Select[Range[0, 1400000], PrimeQ[92*10^#-1 ] &]
PROG
(Magma) [n: n in [0..400] | IsPrime(92*10^n-1)]; // Vincenzo Librandi, Apr 15 2015
(PARI) isok(n) = ispseudoprime(92*10^n-1); \\ Altug Alkan, Apr 18 2018
CROSSREFS
Cf. A002275.
Sequence in context: A298142 A095825 A106359 * A077241 A228469 A066567
KEYWORD
more,hard,nonn
AUTHOR
Robert Price, Apr 14 2015
EXTENSIONS
a(15) from Predrag Kurtovic, May 24 2015
a(16) from Predrag Kurtovic, Apr 18 2018
a(17) from Predrag Kurtovic, Dec 12 2020
a(18) from Predrag Kurtovic, Apr 10 2023
STATUS
approved