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 A102953 Numbers k such that 2*10^k + 4*R_k - 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
 1, 4, 7, 12, 30, 94, 178, 196, 564, 1801, 3520, 3538, 8233, 35161, 37405, 42330, 70051, 90792, 124096, 152670 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers k such that (22*10^k - 13)/9 is prime. a(21) > 2*10^5. - Robert Price, Jun 11 2018 LINKS Table of n, a(n) for n=1..20. Makoto Kamada, Prime numbers of the form 244...443. Index entries for primes involving repunits. FORMULA a(n) = A101958(n) + 1. MATHEMATICA Do[ If[ PrimeQ[(22*10^n - 13)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A101958. Sequence in context: A215329 A208724 A183336 * A111157 A255340 A272251 Adjacent sequences: A102950 A102951 A102952 * A102954 A102955 A102956 KEYWORD more,nonn AUTHOR Robert G. Wilson v, Dec 17 2004 EXTENSIONS a(14)-a(16) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013 a(17)-a(18) from Robert Price, Mar 16 2015 a(19)-a(20) from Robert Price, Jun 11 2018 STATUS approved

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Last modified July 21 00:10 EDT 2024. Contains 374461 sequences. (Running on oeis4.)