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 A111018 Indices of Catalan numbers that are == 0 mod 3 (cf. A000108). 2
 5, 6, 7, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 32, 33, 34, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 86, 87, 88, 95, 96, 97, 98, 99, 100 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS Conjecture: The sequence contains all numbers n such that n and n+1 are both in the sequence A074940, or, equivalently, such that neither n nor n+1 is in the sequence A005836. - L. Edson Jeffery, Dec 02 2015 LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 MATHEMATICA Flatten[Position[CatalanNumber[Range[100]], _?(Divisible[#, 3]&)]] (* Harvey P. Dale, Oct 25 2011 *) Select[Range@100, Divisible[CatalanNumber@#, 3]&] (* Vladimir Reshetnikov, Nov 06 2015 *) (* Conjectured: *) a007814[x_] := IntegerExponent[x, 2]; a003602[x_] := (1 + x/2^a007814[x])/2; a005836[1] := 0; a005836[n_] := If[OddQ[n], 3*a005836[Floor[(n + 1)/2]], a005836[n - 1] + 1]; a265100[n_] := 9*a005836[n] + 5; a265104[n_] := (3/2)*(3^(a007814[n] + 1) - 1) + a265100[2^(a007814[n])*(2*a003602[n] - 1)]; a111018 = {}; Do[AppendTo[a111018, Range[a265100[n], a265104[n] - 1]], {n, 6}]; Take[Flatten[a111018], 66] (* L. Edson Jeffery, Dec 03 2015 *) PROG (PARI) for(n=0, 1e3, if(binomial(2*n, n)/(n+1) % 3 == 0, print1(n, ", "))) \\ Altug Alkan, Dec 02 2015 (MAGMA) [n: n in [1..200] | Binomial(2*n, n) div (n+1) mod 3 eq 0]; // Vincenzo Librandi, Dec 03 2015 CROSSREFS Cf. A000108. Cf. A085296, A265100, A265104. Sequence in context: A067531 A031029 A134985 * A326132 A161925 A078694 Adjacent sequences:  A111015 A111016 A111017 * A111019 A111020 A111021 KEYWORD nonn AUTHOR Robert G. Wilson v, Sep 09 2005 STATUS approved

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Last modified September 28 17:43 EDT 2020. Contains 337393 sequences. (Running on oeis4.)