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 A099412 0 together with numbers n such that 4*R_n - 3 is prime, where R_n = 11...1 is the repunit (A002275) of length n. 2
 0, 2, 4, 11, 28, 55, 94, 475, 2080, 4835, 5845, 12338, 20194, 49529, 56989, 71546, 77309 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also numbers n such that abs((4*10^n-31))/9 is prime. a(18) > 10^5. - Robert Price Sep 06 2014 LINKS Makoto Kamada, Prime numbers of the form 44...441. FORMULA a(n) = A056681(n-1) + 1. MATHEMATICA Do[ If[ PrimeQ[ 4(10^n - 1)/9 - 3], Print[n]], {n, 0, 7000}] PROG (PARI) for(n=0, 10^4, if(ispseudoprime(abs(4*(10^n-1)/9-3)), print1(n, ", "))) \\ Derek Orr, Sep 06 2014 CROSSREFS Cf. A002275, A056681, A104659. Sequence in context: A234844 A205504 A290337 * A100094 A099525 A024742 Adjacent sequences:  A099409 A099410 A099411 * A099413 A099414 A099415 KEYWORD nonn AUTHOR Robert G. Wilson v, Oct 14 2004 EXTENSIONS a(12)-a(17) from Kamada data by Robert Price, Sep 06 2014 STATUS approved

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Last modified April 20 12:07 EDT 2021. Contains 343135 sequences. (Running on oeis4.)