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A202854
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The Matula-Göbel numbers of rooted trees T for which the sequence formed by the number of k-matchings of T (k=0,1,2,...) is palindromic.
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1
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1, 2, 5, 6, 18, 23, 26, 41, 54, 78, 103, 122, 162, 167, 202, 234, 283, 338, 366, 419, 486, 502, 547, 606, 643, 702, 794, 1009, 1014, 1093, 1098, 1346, 1458, 1506, 1543, 1586, 1597, 1818, 1906, 1999, 2106, 2371, 2382, 2462, 2626, 2719, 2962
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OFFSET
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1,2
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COMMENTS
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Alternatively, the Matula-Göbel numbers of rooted trees for which the matching-generating polynomial is palindromic.
A k-matching in a graph is a set of k edges, no two of which have a vertex in common.
The Matula-Göbel number of a rooted tree can be defined in the following recursive manner: to the one-vertex tree there corresponds the number 1; to a tree T with root degree 1 there corresponds the t-th prime number, where t is the Matula-Goebel number of the tree obtained from T by deleting the edge emanating from the root; to a tree T with root degree m>=2 there corresponds the product of the Matula-Göbel numbers of the m branches of T.
After activating the Maple program, the command m(n) will yield the matching-generating polynomial of the rooted tree having Matula-Göbel number n.
The given Maple program gives the required Matula-Göbel numbers up to L=200 (adjustable).
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REFERENCES
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C. D. Godsil, Algebraic Combinatorics, Chapman & Hall, New York, 1993.
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LINKS
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FORMULA
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Define b(n) (c(n)) to be the generating polynomials of the matchings of the rooted tree with Matula-Göbel number n that contain (do not contain) the root, with respect to the size of the matching. We have the following recurrence for the pair M(n)=[b(n),c(n)]. M(1)=[0,1]; if n=p(t) (=the t-th prime), then M(n)=[xc(t),b(t)+c(t)]; if n=rs (r,s,>=2), then M(n)=[b(r)c(s)+c(r)b(s), c(r)c(s)]. Then m(n)=b(n)+c(n) is the generating polynomial of the matchings of the rooted tree with respect to the size of the matchings (called matching-generating polynomial). [The actual matching polynomial is obtained by the substitution x = -1/x^2, followed by multiplication by x^N(n), where N(n) is the number of vertices of the rooted tree.]
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EXAMPLE
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5 is in the sequence because the corresponding rooted tree is a path abcd on 4 vertices. We have 1 0-matching (the empty set), 3 1-matchings (ab), (bc), (cd), and 1 2-matchings (ab, cd). The sequence 1,3,1 is palindromic.
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MAPLE
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L := 200: with(numtheory): N := proc (n) local r, s: r := proc (n) options operator, arrow: op(1, factorset(n)) end proc: s := proc (n) options operator, arrow: n/r(n) end proc: if n = 1 then 1 elif bigomega(n) = 1 then 1+N(pi(n)) else N(r(n))+N(s(n))-1 end if end proc: M := proc (n) local r, s: r := proc (n) options operator, arrow: op(1, factorset(n)) end proc: s := proc (n) options operator, arrow: n/r(n) end proc: if n = 1 then [0, 1] elif bigomega(n) = 1 then [x*M(pi(n))[2], M(pi(n))[1]+M(pi(n))[2]] else [M(r(n))[1]*M(s(n))[2]+M(r(n))[2]*M(s(n))[1], M(r(n))[2]*M(s(n))[2]] end if end proc: m := proc (n) options operator, arrow: sort(expand(M(n)[1]+M(n)[2])) end proc: PAL := {}: for n to L do if m(n) = numer(subs(x = 1/x, m(n))) then PAL := `union`(PAL, {n}) else end if end do: PAL;
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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