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A143454
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Expansion of 1/(x^k*(1 - x - 3*x^(k+1))) for k=3.
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5
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1, 4, 7, 10, 13, 25, 46, 76, 115, 190, 328, 556, 901, 1471, 2455, 4123, 6826, 11239, 18604, 30973, 51451, 85168, 140980, 233899, 388252, 643756, 1066696, 1768393, 2933149, 4864417, 8064505, 13369684, 22169131, 36762382, 60955897, 101064949, 167572342
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OFFSET
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0,2
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COMMENTS
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a(n) is also the number of length n quaternary words with at least 3 0-digits between any other digits.
The compositions of n in which each natural number is colored by one of p different colors are called p-colored compositions of n. For n >= 7, 4*a(n-7) equals the number of 4-colored compositions of n with all parts >= 4, such that no adjacent parts have the same color. - Milan Janjic, Nov 27 2011
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LINKS
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Vincenzo Librandi, Table of n, a(n) for n = 0..1000
D. Birmajer, J. B. Gil, and M. D. Weiner, On the Enumeration of Restricted Words over a Finite Alphabet, J. Int. Seq. 19 (2016) # 16.1.3, Example 9.
Index entries for linear recurrences with constant coefficients, signature (1,0,0,3).
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FORMULA
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G.f.: (1 + 3*x + 3*x^2 + 3*x^3) / (1 - x - 3*x^4). - R. J. Mathar, Aug 04 2019
a(n) = Sum_{j=0..(n+3)/3} 3^j*C(n-3*j+3,j). - Vladimir Kruchinin, May 24 2011
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MAPLE
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a:= proc(k::nonnegint) local n, i, j; if k=0 then unapply(4^n, n) else unapply((Matrix(k+1, (i, j)-> if (i=j-1) or j=1 and i=1 then 1 elif j=1 and i=k+1 then 3 else 0 fi)^(n+k))[1, 1], n) fi end(3): seq(a(n), n=0..50);
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MATHEMATICA
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a[n_]:= Sum[3^j*Binomial[n-3*j+3, j], {j, 0, (n+3)/3}]; Table[a[n], {n, 0, 50}] (* Jean-François Alcover, Feb 04 2014, after Vladimir Kruchinin *)
LinearRecurrence[{1, 0, 0, 3}, {1, 4, 7, 10}, 41] (* G. C. Greubel, May 08 2021 *)
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PROG
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(Maxima) a(n):= sum(3^j*binomial(n-3*j+3, j), j, 0, (n+3)/3); /* Vladimir Kruchinin, May 24 2011 */
(PARI) Vec(1/(x^3*(1-x-3*x^4))+O(x^99)) \\ Charles R Greathouse IV, Sep 26 2012
(PARI) my(p=Mod('x, x^4-'x^3-3)); a(n) = vecsum(Vec(lift(p^(n+3)))); \\ Kevin Ryde, May 11 2021
(Magma) [n le 4 select 3*n-2 else Self(n-1) +3*Self(n-4): n in [1..51]]; // G. C. Greubel, May 08 2021
(Sage)
def a(n): return 3*n+1 if (n<4) else a(n-1) + 3*a(n-4)
[a(n) for n in (0..40)] # G. C. Greubel, May 08 2021
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CROSSREFS
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3rd column of A143461.
Sequence in context: A069212 A091290 A119256 * A318774 A341282 A065810
Adjacent sequences: A143451 A143452 A143453 * A143455 A143456 A143457
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KEYWORD
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nonn,easy
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AUTHOR
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Alois P. Heinz, Aug 16 2008
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STATUS
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approved
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