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 A206499 The sum of the distances between all unordered pairs of branch vertices in the rooted tree with Matula-Goebel number n. A branch vertex is a vertex of degree >=3. 4
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 OFFSET 1,49 COMMENTS The Matula-Goebel 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-Goebel numbers of the m branches of T. The A. Ilic and M. Ilic reference considers the statistic: the sum of the distances between all unordered pairs of vertices of degree k (see A212618, A212619). REFERENCES F. Goebel, On a 1-1 correspondence between rooted trees and natural numbers, J. Combin. Theory, B 29 (1980), 141-143. I. Gutman and A. Ivic, On Matula numbers, Discrete Math., 150, 1996, 131-142. I. Gutman and Y-N. Yeh, Deducing properties of trees from their Matula numbers, Publ. Inst. Math., 53 (67), 1993, 17-22. D. W. Matula, A natural rooted tree enumeration by prime factorization, SIAM Review, 10, 1968, 273. LINKS E. Deutsch, Rooted tree statistics from Matula numbers, arXiv:1111.4288. A. Ilic and M. Ilic, Generalizations of Wiener polarity index and terminal Wiener index, arXiv:11106.2986. FORMULA Let bigomega(n) denote the number of prime divisors of n, counted with multiplicities. Let g(n)=g(n,x) be the generating polynomial of the branch vertices of the rooted tree with Matula-Goebel number n with respect to level. We have a(1) = 0; if n = p(t) (=the t-th prime) and bigomega(t) is not 2, then a(n) = a(t); if n = p(t) (=the t-th prime) and bigomega(t) = 2, then a(n) = a(t) + [dg(t)/dx]_{x=1}; if n = rs with r prime, bigomega(s) =/ 2, then a(n) = a(r) + a(s) + [d[g(r)g(s)]/dx]_{x=1}; if n=rs with r prime, bigomega(s)=2, then a(n)=a(r)+a(s)+ [d[g(r)g(s)]/dx]_{x=1} + [dg(r)/dx]_{x=1} + [dg(s)/dx]_{x=1}. EXAMPLE a(28)=1 because the rooted tree with Matula-Goebel number 28 is the rooted tree obtained by joining the trees I, I, and Y at their roots; it has 2 branch vertices and the distance between them is 1. a(49)=2 because the rooted tree with Matula-Goebel number 49 is the rooted tree obtained by joining two copies of Y at their roots; it has 2 branch vertices and the distance between them is 2. MAPLE with(numtheory): g := 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 elif bigomega(n) = 1 and bigomega(pi(n)) <> 2 then sort(expand(x*g(pi(n)))) elif bigomega(n) = 1 and bigomega(pi(n)) = 2 then sort(expand(x+x*g(pi(n)))) elif bigomega(r(n))+bigomega(s(n)) = 2 then sort(expand(g(r(n))-subs(x = 0, g(r(n)))+g(s(n))-subs(x = 0, g(s(n))))) else sort(expand(g(r(n))-subs(x = 0, g(r(n)))+g(s(n))-subs(x = 0, g(s(n)))+1)) end if end proc: a := 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 elif bigomega(n) = 1 and bigomega(pi(n)) <> 2 then a(pi(n)) elif bigomega(n) = 1 and bigomega(pi(n)) = 2 then a(pi(n))+subs(x = 1, diff(g(pi(n)), x)) elif bigomega(s(n)) = 2 then a(r(n))+a(s(n))+subs(x = 1, diff((1+g(r(n)))*(1+g(s(n))), x)) else a(r(n))+a(s(n))+subs(x = 1, diff(g(r(n))*g(s(n)), x)) end if end proc: seq(a(n), n = 1 .. 120); CROSSREFS Cf. A212618, A212619. Sequence in context: A176891 A219486 A284574 * A277885 A109527 A186715 Adjacent sequences:  A206496 A206497 A206498 * A206500 A206501 A206502 KEYWORD nonn AUTHOR Emeric Deutsch, May 22 2012 STATUS approved

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Last modified October 15 10:15 EDT 2019. Contains 328026 sequences. (Running on oeis4.)