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 A103106 Numbers n such that 9*10^n + 7*R_n - 4 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
 0, 6, 15, 36, 45, 447, 1031, 1239, 3492, 6069, 13647, 17238, 56271, 72711, 75381 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (88*10^n-43)/9 is prime. a(16) > 10^5. - Robert Price, Nov 12 2015 LINKS Table of n, a(n) for n=1..15. Makoto Kamada, Prime numbers of the form 977...773. Index entries for primes involving repunits. FORMULA a(n) = A101015(n-1) + 1, for n>1. MATHEMATICA Do[ If[ PrimeQ[(88*10^n - 43)/9], Print[n]], {n, 0, 10000}] PROG (PARI) is(n)=isprime(9*10^n + 7*((10^n-1)/9) - 4) \\ Anders Hellström, Nov 12 2015 CROSSREFS Cf. A002275, A101015. Sequence in context: A338057 A191002 A127469 * A272788 A273151 A320941 Adjacent sequences: A103103 A103104 A103105 * A103107 A103108 A103109 KEYWORD more,nonn AUTHOR Robert G. Wilson v, Jan 19 2005 EXTENSIONS a(11)-a(12) from Kamada data by Robert Price, Dec 14 2010 Inserted a(1)=0 and added a(13)-a(15) from Robert Price, Nov 12 2015 STATUS approved

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Last modified April 23 07:57 EDT 2024. Contains 371905 sequences. (Running on oeis4.)