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A001638
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A Fielder sequence: a(n) = a(n-1) + a(n-3) + a(n-4), n >= 4.
(Formerly M3351 N1348)
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14
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4, 1, 1, 4, 9, 11, 16, 29, 49, 76, 121, 199, 324, 521, 841, 1364, 2209, 3571, 5776, 9349, 15129, 24476, 39601, 64079, 103684, 167761, 271441, 439204, 710649, 1149851, 1860496, 3010349, 4870849, 7881196, 12752041, 20633239, 33385284, 54018521, 87403801
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OFFSET
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0,1
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COMMENTS
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For n > 1, a(n) is the number of ways of choosing a subset of vertices of an n-cycle so that every vertex of the n-cycle is adjacent to one of the chosen vertices. (Note that this is not the same as the number of dominating sets of the n-cycle, which is given by A001644.) - Joel B. Lewis, Sep 12 2010
For n >= 3, a(n) is also the number of total dominating sets in the n-cycle graph. - Eric W. Weisstein, Apr 10 2018
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REFERENCES
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N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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6, 2006, pp. 840-855.
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FORMULA
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G.f.: (1-x)*(4+x+x^2)/((1+x^2)*(1-x-x^2)).
a(n) = L(n) + i^n + (-i)^n, a(2n) = L(n)^2, a(2n+1) = L(2n+1) where L() is Lucas sequence A000032.
a(n) = Trace(M^n), where M = [0, 0, 0, 1; 1, 0, 0, 1; 0, 1, 0, 0; 0, 0, 1, 1] is the companion matrix to the monic polynomial x^4 - x^3 - x - 1. . It follows that the sequence satisfies the Gauss congruences: a(n*p^r) == a(n*p^(r-1)) (mod p^r) for positive integers n and r and all primes p. See Zarelua. - Peter Bala, Jan 08 2023
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MAPLE
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A001638:=-(z+1)*(4*z**2-z+1)/(z**2+z-1)/(z**2+1); # conjectured by Simon Plouffe in his 1992 dissertation; gives sequence except for the initial 4
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MATHEMATICA
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LinearRecurrence[{1, 0, 1, 1}, {4, 1, 1, 4}, 50] (* T. D. Noe, Aug 09 2012 *)
CoefficientList[Series[(-4 + 3 x + x^3)/(-1 + x + x^3 + x^4), {x, 0, 20}], x] (* Eric W. Weisstein, Apr 10 2018 *)
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PROG
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(PARI) a(n)=if(n<0, 0, fibonacci(n+1)+fibonacci(n-1)+simplify(I^n+(-I)^n))
(PARI) a(n)=if(n<0, 0, polsym((1+x-x^2)*(1+x^2), n)[n+1])
(Magma) I:=[4, 1, 1, 4]; [n le 4 select I[n] else Self(n-1) + Self(n-3) + Self(n-4): n in [1..30]]; // G. C. Greubel, Jan 09 2018
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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