

A143940


Triangle read by rows: T(n,k) is the number of unordered pairs of vertices at distance k in a linear chain of n triangles (i.e., joined like VVV..VV; here V is a triangle!), 1 <= k <= n.


1



3, 6, 4, 9, 8, 4, 12, 12, 8, 4, 15, 16, 12, 8, 4, 18, 20, 16, 12, 8, 4, 21, 24, 20, 16, 12, 8, 4, 24, 28, 24, 20, 16, 12, 8, 4, 27, 32, 28, 24, 20, 16, 12, 8, 4, 30, 36, 32, 28, 24, 20, 16, 12, 8, 4, 33, 40, 36, 32, 28, 24, 20, 16, 12, 8, 4, 36, 44, 40, 36, 32, 28, 24, 20, 16, 12, 8, 4
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OFFSET

1,1


COMMENTS

The entries in row n are the coefficients of the Wiener polynomial of a linear chain of n triangles.
Sum of entries in row n = n(2n+1) = A014105(n).
Sum_{k=1..n} k*T(n,k) = the Wiener index of the linear chain of n triangles = A143941(n).


LINKS

Table of n, a(n) for n=1..78.
B. E. Sagan, YN. Yeh and P. Zhang, The Wiener Polynomial of a Graph, Internat. J. of Quantum Chem., 60, 1996, 959969.


FORMULA

T(n,1)=3n; T(n,k) = 4(nk+1) for k>1.
G.f. = G(q,z) = qz/(3+qz)/((1qz)*(1z)^2).


EXAMPLE

T(2,1)=6 because the chain of 2 triangles has 6 edges.
Triangle starts:
3;
6, 4;
9, 8, 4;
12, 12, 8, 4;
15, 16, 12, 8, 4;


MAPLE

T:=proc(n, k) if n < k then 0 elif k = 1 then 3*n else 4*n4*k+4 end if end proc: for n to 12 do seq(T(n, k), k=1..n) end do; # yields sequence in triangular form


CROSSREFS

Cf. A014105, A143941.
Sequence in context: A083682 A021278 A336761 * A321773 A083349 A065230
Adjacent sequences: A143937 A143938 A143939 * A143941 A143942 A143943


KEYWORD

nonn,tabl


AUTHOR

Emeric Deutsch, Sep 06 2008


STATUS

approved



