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A054878
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Number of closed walks of length n along the edges of a tetrahedron based at a vertex.
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23
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1, 0, 3, 6, 21, 60, 183, 546, 1641, 4920, 14763, 44286, 132861, 398580, 1195743, 3587226, 10761681, 32285040, 96855123, 290565366, 871696101, 2615088300, 7845264903, 23535794706, 70607384121, 211822152360, 635466457083
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OFFSET
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0,3
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COMMENTS
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Number of closed walks of length n at a vertex of C_4, the cyclic graph on 4 nodes. 3*A015518(n) + a(n) = 3^n. - Paul Barry, Feb 03 2004
Form the digraph with matrix A = [0,1,1,1; 1,0,1,1; 1,1,0,1; 1,0,1,1]; a(n) corresponds to the (1,1) term of A^n. - Paul Barry, Oct 02 2004
Absolute values of A084567 (compare generating functions).
For n > 1, 4*a(n)=A218034(n)= the trace of the n-th power of the adjacency matrix for a complete 4-graph, a 4 X 4 matrix with a null diagonal and all ones for off-diagonal elements. The diagonal elements for the n-th power are a(n) and the off-diagonal are a(n)+1 for an odd power and a(n)-1 for an even (cf. A001045). - Tom Copeland, Nov 06 2012
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LINKS
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FORMULA
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a(n) = (3^n + (-1)^n*3)/4.
G.f.: 1/4*(3/(1+x) + 1/(1-3*x)).
E.g.f.: (exp(3*x) + 3*exp(-x))/4. - Paul Barry, Apr 20 2003
a(n) = 3^n - a(n-1) with a(0)=0. - Labos Elemer, Apr 26 2003
G.f.: (1 - 3*x^2 - 2*x^3)/(1 - 6*x^2 - 8*x^3 - 3*x^4) = (1 - 3*x^2 - 2*x^3)/charpoly(adj(C_4)). - Paul Barry, Feb 03 2004
G.f.: (1-2*x)/(1 - 2*x - 3*x^2).
a(n) = 2*a(n-1) + 3*a(n-2). (End)
G.f.: 1 - x + x/Q(0), where Q(k) = 1 + 3*x^2 - (3*k+4)*x + x*(3*k+1 - 3*x)/Q(k+1); (continued fraction). - Sergei N. Gladkovskii, Oct 07 2013
a(n+m) = a(n)*a(m) + a(n+1)*a(m+1)/3. - Yuchun Ji, Sep 12 2017
a(n) = 3*a(n-1) + 3*(-1)^n. - Yuchun Ji, Sep 13 2017
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MAPLE
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MATHEMATICA
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Table[(3^n + (-1)^n*3)/4, {n, 0, 26}] (* or *)
CoefficientList[Series[1/4*(3/(1 + x) + 1/(1 - 3 x)), {x, 0, 26}], x] (* Michael De Vlieger, Sep 15 2017 *)
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PROG
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(PARI) a(n) = (3^n + 3*(-1)^n)/4; \\ Altug Alkan, Sep 17 2017
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CROSSREFS
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{a(n)/3} for n>0 is A015518, non-closed walks.
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KEYWORD
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nonn,walk,easy
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AUTHOR
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Paolo Dominici (pl.dm(AT)libero.it), May 23 2000
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STATUS
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approved
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