This site is supported by donations to The OEIS Foundation.

Thanks to everyone who made a donation during our annual appeal!
To see the list of donors, or make a donation, see the OEIS Foundation home page.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A193402 The Matula-Göbel numbers of the rooted trees such that 2 is an eigenvalue of the Laplacian matrix. 2
 2, 5, 6, 15, 18, 22, 23, 26, 31, 41, 45, 54, 55, 65, 66, 69, 78, 93, 94, 103, 122, 123, 135, 137, 158, 162, 165, 166, 167, 195, 198, 202, 207, 211, 234, 235, 242, 253, 254, 279, 282, 283, 286, 299, 305, 309, 338, 341, 343, 358, 366, 369, 394, 395, 401, 403 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS 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-Göbel 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. LINKS E. Deutsch, Rooted tree statistics from Matula numbers, arXiv:1111.4288 [math.CO], 2011. Yi-zheng Fan, On the eigenvalue two and matching number of a tree, Acta Math. Appl. Sinica, English Series, 20, 2004, 257-262. F. Göbel, 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 Yeong-Nan Yeh, Deducing properties of trees from their Matula numbers, Publ. Inst. Math., 53 (67), 1993, 17-22. D. Matula, A natural rooted tree enumeration by prime factorization, SIAM Rev. 10 (1968) 273. FORMULA Let T be a rooted tree with root b. If b has degree 1, then let A be the rooted tree with root c, obtained from T by deleting the edge bc emanating from the root. If b has degree >=2, then A is obtained (not necessarily in a unique way) by joining at b two trees B and C, rooted at b. It is straightforward to express the distance matrix of T in terms of the entries of the distance matrix of A (resp. of B and C). Making use of this, the Maple program (improvable!) finds recursively the distance matrices of the rooted trees with Matula-Göbel numbers 1..1000 (upper limit can be altered), then switches (easily) to the Laplacian matrices and finds their  characteristic polynomials. EXAMPLE 5 is in the sequence because the rooted tree with Matula-Göbel number 5 is the path on 4 vertices; the Laplacian matrix is [1,-1,0,0; -1,2,-1,0; 0,-1,2,-1;0,0,-1,1] with characteristic polynomial x(x-2)(x^2 -4x +2). MAPLE with(numtheory): with(linalg): with(LinearAlgebra): DA := proc (d) local aa: aa := proc (i, j) if d[i, j] = 1 then 1 else 0 end if end proc: Matrix(RowDimension(d), RowDimension(d), aa) end proc: AL := proc (a) local ll: ll := proc (i, j) if i = j then add(a[i, k], k = 1 .. RowDimension(a)) else -a[i, j] end if end proc: Matrix(RowDimension(a), RowDimension(a), ll) end proc: V := 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+V(pi(n)) else V(r(n))+V(s(n))-1 end if end proc: d := proc (n) local r, s, C, a: r := proc (n) options operator, arrow: op(1, factorset(n)) end proc: s := proc (n) options operator, arrow: n/r(n) end proc: C := proc (A, B) local c: c := proc (i, j) options operator, arrow: A[1, i]+B[1, j+1] end proc: Matrix(RowDimension(A), RowDimension(B)-1, c) end proc: a := proc (i, j) if i = 1 and j = 1 then 0 elif 2 <= i and 2 <= j then dd[pi(n)][i-1, j-1] elif i = 1 then 1+dd[pi(n)][1, j-1] elif j = 1 then 1+dd[pi(n)][i-1, 1] else  end if end proc: if n = 1 then Matrix(1, 1, [0]) elif bigomega(n) = 1 then Matrix(V(n), V(n), a) else Matrix(blockmatrix(2, 2, [dd[r(n)], C(dd[r(n)], dd[s(n)]), Transpose(C(dd[r(n)], dd[s(n)])), SubMatrix(dd[s(n)], 2 .. RowDimension(dd[s(n)]), 2 .. RowDimension(dd[s(n)]))])) end if end proc: for n to 1000 do dd[n] := d(n) end do: S := {}: for n to 500 do if subs(x = 2, CharacteristicPolynomial(AL(DA(d(n))), x)) = 0 then S := `union`(S, {n}) else  end if end do: S; CROSSREFS Cf. A061775, A184187, A193403, A193401. Sequence in context: A205385 A287203 A291211 * A193405 A037079 A101325 Adjacent sequences:  A193399 A193400 A193401 * A193403 A193404 A193405 KEYWORD nonn AUTHOR Emeric Deutsch, Feb 11 2012 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified January 17 14:12 EST 2019. Contains 319225 sequences. (Running on oeis4.)