OFFSET
1,1
COMMENTS
a(n) = Sum(k*A192024(n,k),k>=1).
LINKS
T. Mansour, M. Schork, The vertex PI index and Szeged index of bridge graphs, Discrete Appl. Math., 157, 2009, 1600-1606 (see last page).
Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).
FORMULA
a(n) = n*(3*n^2+12*n-7)/2.
G.f.: x*(4+13*x-8*x^2)/(1-x)^4.
EXAMPLE
a(2)=29 because in the graph \/_\/ there are 5 pairs of nodes at distance 1, 6 pairs at distance 2, and 4 pairs at distance 3 (5*1 + 6*2 + 4*3 = 29).
MAPLE
a := n -> (1/2)*n*(3*n^2+12*n-7): seq(a(n), n = 1 .. 40);
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch, Jun 25 2011
STATUS
approved