A181773 Molecular topological indices of the cocktail party graphs.

0, 48, 240, 672, 1440, 2640, 4368, 6720, 9792, 13680, 18480, 24288, 31200, 39312, 48720, 59520, 71808, 85680, 101232, 118560, 137760, 158928, 182160, 207552, 235200, 265200, 297648, 332640, 370272, 410640

Offset: 1

Author

Eric W. Weisstein, Jul 10 2011

Keywords

• nonn
• easy

Comments

a(n) is the number of 2 X 2 matrices (all four elements distinct) having entries in {-n,...,0,...,n} with determinant equal to the permanent. - Indranil Ghosh, Dec 25 2016

Links

• Eric Weisstein's World of Mathematics, Molecular Topological Index
• Index entries for linear recurrences with constant coefficients, signature (4,-6,4,-1).

Crossrefs

Cf. A280059 (2 X 2 matrices, elements can be repeated).

Programs

Maple

A181773:=n->8*(n-1)*n*(2*n-1): seq(A181773(n), n=1..50); # Wesley Ivan Hurt, Apr 11 2017

Formula

a(n) = 8*(n-1)*n*(2n-1).

a(n) = 16*A059270(n-1).

G.f.: 48*x^2*(x+1)/(x-1)^4. - Colin Barker, Oct 17 2012

a(n) = 48*A000330(n-1). - R. J. Mathar, Jan 04 2017

From Omar E. Pol, Jan 05 2017: (Start)

a(n) = 24*A006331(n-1) = 12*A002492(n-1) = 8*A055112(n-1).

a(n) = 2*A069074(n-2), n >= 2. (End)