OFFSET
1,2
COMMENTS
Except for the first term of 0, a(n) is the set of all integers k such that 6k+12 is a perfect square. - Gary Detlefs, Mar 01 2010
For n > 2, the surface area of a rectangular prism with sides n-2, n-1, and n. - J. M. Bergot, Sep 12 2011
Also the number of 4-cycles in the (n+2) X (n+2) knight graph. - Eric W. Weisstein, May 05 2017
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..1000
Eric Weisstein's World of Mathematics, Graph Cycle.
Eric Weisstein's World of Mathematics, Knight Graph.
Index entries for linear recurrences with constant coefficients, signature (3,-3,1).
FORMULA
a(n) = 6*n^2 - 12*n + 4, n > 1.
From Colin Barker, Sep 06 2013: (Start)
a(n) = 3*a(n-1) - 3*a(n-2) + a(n-3) for n > 4.
G.f.: 2*x^2*(x^2-5*x-2) / (x-1)^3. (End)
a(n+1) = (n*i-1)^3 - (n*i+1)^3, where n > 0, i=sqrt(-1). - Bruno Berselli, Jan 23 2014
E.g.f.: 2*((3*x^2 - 3*x + 2)*exp(x) + x - 2). - G. C. Greubel, Dec 23 2016
From Amiram Eldar, Aug 20 2022: (Start)
Sum_{n>=2} 1/a(n) = 1/4 - cot(Pi/sqrt(3))*Pi/(4*sqrt(3)).
Sum_{n>=2} (-1)^n/a(n) = cosec(Pi/sqrt(3))*Pi/(4*sqrt(3)) - 1/4. (End)
EXAMPLE
a(2)=4: the four rotations of the (connected) L tromino.
MAPLE
MATHEMATICA
CoefficientList[Series[(2*z*(z^3 - 5*z^2 - 2*z))/(z - 1)^3, {z, 0, 100}], z] (* Vladimir Joseph Stephan Orlovsky, Jul 17 2011 *)
Join[{0}, Table[6*n^2 - 12*n + 4, {n, 2, 50}]] (* G. C. Greubel, Dec 23 2016 *)
Join[{0}, LinearRecurrence[{3, -3, 1}, {4, 22, 52}, 50]] (* G. C. Greubel, Dec 23 2016 *)
Length /@ Table[FindCycle[KnightTourGraph[n + 2, n + 2], {4}, All], {n, 20}] (* Eric W. Weisstein, May 05 2017 *)
PROG
(PARI) concat([0], Vec(2*x^2*(x^2-5*x-2) / (x-1)^3 + O(x^50))) \\ G. C. Greubel, Dec 23 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
David Bevan, Jul 28 2009
STATUS
approved