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A098959
Numbers k such that 2*10^k + 6*R_k - 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 2, 3, 5, 6, 7, 11, 21, 30, 68, 73, 169, 176, 345, 823, 1021, 1191, 2073, 2755, 10717, 14673, 16754, 17606, 81029, 120851, 167965, 200408
OFFSET
1,2
COMMENTS
Also numbers k such that (8*10^k - 11)/3 is prime.
a(28) > 3*10^5. - Robert Price, Jul 13 2023
FORMULA
a(n) = A101964(n) + 1.
EXAMPLE
For k = 1, 2, 3, 5, 6, 7, we get 23, 263, 2663, 266663, 2666663 and 26666663 which are primes.
MATHEMATICA
Do[ If[ PrimeQ[(8*10^n - 11)/3], Print[n]], {n, 0, 10000}]
CROSSREFS
Sequence in context: A062084 A331394 A273524 * A071251 A077571 A101730
KEYWORD
more,nonn
AUTHOR
Julien Peter Benney (jpbenney(AT)ftml.net), Oct 21 2004
EXTENSIONS
a(15), a(16) & a(17) from Ray Chandler, Nov 04 2004
a(18) & a(19) from Robert G. Wilson v, Dec 17 2004
a(20)-a(23) from Kamada link by Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 02 2008
a(24) from Kamada data by Robert Price, Jan 17 2015
a(25)-a(27) from Robert Price, Jul 13 2023
STATUS
approved