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 A089675 Numbers n such that 9*R_n - 2 is a prime number, where R_n = 11...1 is the repunit (A002275) of length n. 18
 1, 2, 3, 17, 140, 990, 1887, 3530, 5996, 13820, 21873, 26045, 87720, 232599 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that 10^n - 3 is prime. If n is in the sequence (10^n-3 is prime) and m=3*(10^n-3) then phi(m)=reversal(m), i.e., m is in the sequence A069215. - Farideh Firoozbakht, Dec 25 2004 No further terms for n<=407197, see Kamada link. LINKS Makoto Kamada, Prime numbers of the form 99...997. FORMULA a(n) = A056662(n) + 1 EXAMPLE 10^2 - 3 = 97 is a prime number (in fact all terms are the largest less than 10^n). MATHEMATICA m = 1000; For[n = 1, n < m, If[PrimeQ[10^n - 3], Print[n]]; n++] CROSSREFS Cf. A002275, A056662, A069215, A093172, A101700. Sequence in context: A164816 A259535 A042978 * A041383 A042903 A132534 Adjacent sequences:  A089672 A089673 A089674 * A089676 A089677 A089678 KEYWORD more,nonn AUTHOR Michael Gottlieb (mzrg(AT)verizon.net), Jan 05 2004 EXTENSIONS a(8) from Robert G. Wilson v, Jan 14 2004 a(9) and a(10) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 06 2004 a(11) from Gabriel Cunningham (gcasey(AT)mit.edu), Mar 13 2004 a(12) from Henri Lifchitz. Edited by Patrick De Geest, Dec 28 2004 Edited by Ray Chandler, Dec 23 2010 STATUS approved

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Last modified November 15 01:39 EST 2018. Contains 317224 sequences. (Running on oeis4.)