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 A123347 Number of words of length n over the alphabet {1,2,3,4,5} such that 1 is not followed by an odd letter. 9
 1, 5, 22, 98, 436, 1940, 8632, 38408, 170896, 760400, 3383392, 15054368, 66984256, 298045760, 1326151552, 5900697728, 26255094016, 116821771520, 519797274112, 2312832639488, 10290925106176, 45789365703680, 203739313027072, 906535983515648, 4033622560116736 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS Appears to be Kekulé numbers for certain benzenoids (see the Cyvin-Gutman book for details). REFERENCES S. J. Cyvin and I. Gutman, Kekulé structures in benzenoid hydrocarbons, Lecture Notes in Chemistry, No. 46, Springer, New York, 1988 (see p. 78). LINKS Andrew Howroyd, Table of n, a(n) for n = 0..500 D. Birmajer, J. B. Gil, M. D. Weiner, n the Enumeration of Restricted Words over a Finite Alphabet, J. Int. Seq. 19 (2016) # 16.1.3, example 17 Index entries for linear recurrences with constant coefficients, signature (4, 2). FORMULA From Klaus Brockhaus, Oct 03 2009: (Start) Inverse binomial transform of A138395. a(n) = ((2+sqrt(6))^(n+1) + (2-sqrt(6))^(n+1))/4. a(n) = 4*a(n-1) + 2*a(n-2) for n > 1. G.f.: (1 + x)/(1 - 4*x - 2*x^2). (End) a(n) = A090017(n+1)+A090017(n). - R. J. Mathar, Aug 04 2019 EXAMPLE a(2) = 22 because all 25 words of length 2 are included except 11, 13 and 15. MAPLE seq(coeff(series((1+x)/(1-4*x-2*x^2), x, n+1), x, n), n = 0 .. 25); # Muniru A Asiru, Nov 27 2018 MATHEMATICA LinearRecurrence[{4, 2}, {1, 5}, 30] (* Amiram Eldar, Nov 26 2018 *) PROG (PARI) Vec((1 + x)/(1 - 4*x - 2*x^2) + O(x^30)) \\ Andrew Howroyd, Nov 25 2018 (MAGMA) I:=[1, 5]; [n le 2 select I[n] else 4*Self(n-1) + 2*Self(n-2): n in [1..30]]; // G. C. Greubel, Nov 29 2018 (Sage) s=((1+x)/(1-4*x-2*x^2)).series(x, 50); s.coefficients(x, sparse=False) # G. C. Greubel, Nov 29 2018 CROSSREFS Cf. A138395. Sequence in context: A297333 A129158 A129164 * A087439 A033452 A295519 Adjacent sequences:  A123344 A123345 A123346 * A123348 A123349 A123350 KEYWORD nonn,easy AUTHOR N. J. A. Sloane, Oct 10 2006 EXTENSIONS Edited and new name by Armend Shabani and Andrew Howroyd, Nov 25 2018 STATUS approved

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Last modified February 19 19:06 EST 2020. Contains 332047 sequences. (Running on oeis4.)