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 A122946 a(0)=a(1)=0, a(2)=2; for n >= 3, a(n) = a(n-1) + 4*a(n-3). 3
 0, 0, 2, 2, 2, 10, 18, 26, 66, 138, 242, 506, 1058, 2026, 4050, 8282, 16386, 32586, 65714, 131258, 261602, 524458, 1049490, 2095898, 4193730, 8391690, 16775282, 33550202, 67116962, 134218090, 268418898, 536886746, 1073759106, 2147434698, 4294981682, 8590018106 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,3 COMMENTS See lemma 5.2 of Reznick's preprint. Conjecture: count of even Markov numbers in generation n (with generations 0, 1 and 2 labeled as {5}, {13, 29} and {34, 194, 433, 169}. (Checked up to generation 20.) - Wouter Meeussen, Jan 16 2024 Wouter Meeussen's conjecture is true. Proof: label the Markov tree with Markov triples according to the scheme described at A368546. Mod 2, the triples are: row 0: (1,1,0); row 1: (1,1,1), (1,1,0); row 2: (1,0,1), (1,0,1), (1,1,1), (1,1,0); row 3: (1,1,0), (0,1,1), (1,1,0), (0,1,1), (1,0,1), (1,0,1), (1,1,1), (1,1,0); etc. Note that the Markov number labels of the tree (the center numbers of the triples) in rows 0 and 1 include no even numbers, while those in row 2 include two even numbers. Observing that the second triple in row 1 and the first four triples in row 3 are the same or the reverse of the root triple, and noting that every vertex in row 3 and beyond is in a subtree with one of these triples as root, the recurrence follows. - William P. Orrick, Mar 05 2024 LINKS A.H.M. Smeets, Table of n, a(n) for n = 0..300 Bruce Reznick, Regularity properties of the Stern enumeration of the rationals, arXiv:math/0610601 [math.NT], 2006. Index entries for linear recurrences with constant coefficients, signature (1,0,4). FORMULA a(n) = (1/7)*2^(-2 + n/2)*(7*2^(n/2) - 7*cos(n*(Pi - arctan(sqrt(7)))) + 5*sqrt(7)*sin(n*(-Pi + arctan(sqrt(7))))). - Zak Seidov, Oct 26 2006 G.f.: 2*x^2 / ((1-2*x)*(2*x^2+x+1)). - Colin Barker, Jun 20 2013 a(n) = 2 * A089977(n-2) for n >= 2. - Alois P. Heinz, Jan 16 2024 From A.H.M. Smeets, Jan 16 2024: (Start) Limit_{n -> oo} a(n)/a(n-1) = 2. a(n) = 2^(n-2) + A110512(n-2), for n >= 2. (End) MATHEMATICA LinearRecurrence[{1, 0, 4}, {0, 0, 2}, 36] (* James C. McMahon, Jan 16 2024 *) PROG (PARI) a0=a1=0; a2=2; for(n=3, 50, a3=a2+4*a0; a0=a1; a1=a2; a2=a3; print1(a3, ", ")) CROSSREFS Cf. A002487, A002559, A089977, A110512, A368546. Sequence in context: A254706 A353261 A291666 * A087628 A263060 A089871 Adjacent sequences: A122943 A122944 A122945 * A122947 A122948 A122949 KEYWORD nonn,easy AUTHOR Benoit Cloitre, Oct 24 2006 EXTENSIONS Entries checked by Zak Seidov, Oct 26 2006 STATUS approved

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Last modified May 29 14:43 EDT 2024. Contains 372952 sequences. (Running on oeis4.)