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 A056707 Numbers k such that 4*10^k + 3*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 2
 1, 2, 16, 31, 37, 55, 62, 172, 174, 197, 727, 1246, 1752, 4318, 4328, 4930, 6450, 15206, 24041, 25321, 33068, 41484, 51511 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers k such that (13*10^k - 1)/3 is prime. a(24) > 10^5. - Robert Price, Dec 21 2014 LINKS Makoto Kamada, Prime numbers of the form 433...33. EXAMPLE 43 and 433 are prime, so 1 and 2 are terms of the sequence. MATHEMATICA Do[ If[ PrimeQ[ 4*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A093673. Sequence in context: A242933 A321308 A109210 * A069256 A344016 A279034 Adjacent sequences:  A056704 A056705 A056706 * A056708 A056709 A056710 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS 26044 from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(18)-a(23) from Robert Price, Dec 21 2014 STATUS approved

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Last modified July 2 07:50 EDT 2022. Contains 354985 sequences. (Running on oeis4.)