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 A056704 Numbers k such that 3*10^k + 1*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k. 1
 0, 1, 2, 5, 10, 11, 13, 34, 47, 52, 77, 88, 554, 580, 1310, 1505, 8537, 15892, 24022, 27041, 37922, 40033, 134122, 165358, 183760 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS Also numbers k such that (28*10^k - 1)/9 is prime. Although perhaps a degenerate case, A002275 defines R(0)=0.  Thus zero belongs in this sequence since 3*10^0 + 0 = 3*1 + 0 = 3 is prime. - Robert Price, Oct 28 2014 a(26) > 2*10^5. - Robert Price, Dec 19 2014 LINKS Makoto Kamada, Prime numbers of the form 311...11. MATHEMATICA Do[ If[ PrimeQ[ 3*10^n + (10^n-1)/9], Print[n]], {n, 0, 10000}] CROSSREFS Cf. A002275, A068813. Sequence in context: A136817 A140180 A175324 * A230408 A163624 A257031 Adjacent sequences:  A056701 A056702 A056703 * A056705 A056706 A056707 KEYWORD hard,nonn AUTHOR Robert G. Wilson v, Aug 10 2000 EXTENSIONS Added zero by Robert Price, Oct 28 2014 a(18)-a(25) from Kamada data by Robert Price, Dec 19 2014 STATUS approved

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Last modified July 18 15:44 EDT 2019. Contains 325144 sequences. (Running on oeis4.)