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 A056720 Numbers n such that 7*10^n + 3*R_n is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
 0, 1, 2, 3, 5, 53, 56, 343, 908, 1079, 2204, 2379, 9134, 9371, 9728, 11039, 11247, 13275, 16781, 20138, 75368, 87863 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers n such that (22*10^n-1)/3 is prime. a(23) > 10^5. - Robert Price, Dec 26 2014 LINKS Makoto Kamada, Prime numbers of the form 733...33. MATHEMATICA Do[ If[ PrimeQ[ 7*10^n + 3*(10^n-1)/9], Print[n]], {n, 0, 5000}] CROSSREFS Cf. A002275, A093675. Sequence in context: A062608 A041791 A322947 * A100850 A060085 A114370 Adjacent sequences:  A056717 A056718 A056719 * A056721 A056722 A056723 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 11 2000 EXTENSIONS More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008 a(16)-a(22) from Robert Price, Dec 26 2014 STATUS approved

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Last modified April 24 04:23 EDT 2019. Contains 322406 sequences. (Running on oeis4.)