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 A103031 Numbers n such that 6*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 1
 0, 3, 15, 339 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that (56*10^n+61)/9 is prime. a(5) > 10^5. - Robert Price, Sep 07 2015 LINKS Makoto Kamada, Prime numbers of the form 622...229. FORMULA a(n) = A101524(n-1) + 1 for n>1. EXAMPLE For n=0, (56*10^n+61)/9=13 which is prime, so 0 is in the sequence. MATHEMATICA Do[ If[ PrimeQ[(56*10^n + 61)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A101524. Sequence in context: A036279 A156769 A029758 * A012474 A296940 A193119 Adjacent sequences:  A103028 A103029 A103030 * A103032 A103033 A103034 KEYWORD bref,more,nonn,changed AUTHOR Robert G. Wilson v, Jan 18 2005 EXTENSIONS Inserted a(1)=0 by Robert Price, Sep 07 2015 STATUS approved

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Last modified January 21 13:07 EST 2019. Contains 319350 sequences. (Running on oeis4.)