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 A143370 Triangle read by rows: T(n,k) is the number of unordered pairs of vertices at distance k in the grid P_2 x P_n (1 <= k <= n). P_m is the path graph on m vertices. 1
 1, 4, 2, 7, 6, 2, 10, 10, 6, 2, 13, 14, 10, 6, 2, 16, 18, 14, 10, 6, 2, 19, 22, 18, 14, 10, 6, 2, 22, 26, 22, 18, 14, 10, 6, 2, 25, 30, 26, 22, 18, 14, 10, 6, 2, 28, 34, 30, 26, 22, 18, 14, 10, 6, 2, 31, 38, 34, 30, 26, 22, 18, 14, 10, 6, 2, 34, 42, 38, 34, 30, 26, 22, 18, 14, 10, 6, 2 (list; table; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Sum of entries in row n = n(2n-1) = A000384(n). The entries in row n are the coefficients of the Wiener polynomial of the grid P_2 x P_n. Sum_{k=1..n} k*T(n,k) = A131423(n) = the Wiener index of the grid P_2 x P_n. The average of all distances in the grid P_2 x P_n is (n+2)/3. LINKS Table of n, a(n) for n=1..78. B. E. Sagan, Y-N. Yeh and P. Zhang, The Wiener Polynomial of a Graph, Internat. J. of Quantum Chem., 60, 1996, 959-969. FORMULA G.f. = G(q,z) = qz(1+2z+qz)/((1-qz)(1-z)^2). EXAMPLE T(2,1)=4 because in the graph P_2 x P_2 (a square) we have 4 distances equal to 1. Triangle starts: 1; 4, 2; 7, 6, 2; 10, 10, 6, 2; 13, 14, 10, 6, 2; MAPLE G:=q*z*(1+2*z+q*z)/((1-z)^2*(1-q*z)): Gser:= simplify(series(G, z=0, 15)): for n to 12 do p[n]:=sort(coeff(Gser, z, n)) end do: for n to 12 do seq(coeff(p[n], q, j), j=1..n) end do; # yields sequence in triangular form CROSSREFS Cf. A000384. Sequence in context: A002560 A124908 A260593 * A367832 A307869 A016695 Adjacent sequences: A143367 A143368 A143369 * A143371 A143372 A143373 KEYWORD nonn,tabl AUTHOR Emeric Deutsch, Sep 05 2008 STATUS approved

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Last modified July 12 10:18 EDT 2024. Contains 374244 sequences. (Running on oeis4.)