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A243645 Number of ways two L-tiles can be placed on an n X n square. 3
0, 0, 0, 1, 20, 87, 244, 545, 1056, 1855, 3032, 4689, 6940, 9911, 13740, 18577, 24584, 31935, 40816, 51425, 63972, 78679, 95780, 115521, 138160, 163967, 193224, 226225, 263276, 304695, 350812, 401969, 458520, 520831, 589280, 664257, 746164, 835415, 932436 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

This sequence also represents the number of edges added to G so that it is complete, where G is a graph of (n-1)^2 nodes arranged in a rhombus and embedded in the hexagonal lattice. G begins with A045944(n-2) edges and a(n) edges are added to form a complete graph. - John Tyler Rascoe, Sep 24 2022

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..1000

Index entries for linear recurrences with constant coefficients, signature (5,-10,10,-5,1).

FORMULA

G.f.: x^3*(x^3+3*x^2-15*x-1) / (x-1)^5.

a(n) = (n^4-4*n^3-n^2+18*n-16)/2 for n>=2, a(n) = 0 for n<2.

a(n) = A083374(n-1) - A045944(n-2) for n>=2. - John Tyler Rascoe, Sep 24 2022

EXAMPLE

a(3) = 1:

._____.

|_| |_|

| |___|

|___|_| .

MAPLE

a:= n-> `if`(n<2, 0, ((((n-4)*n-1)*n+18)*n-16)/2):

seq(a(n), n=0..50);

MATHEMATICA

CoefficientList[Series[x^3 (x^3+3x^2-15x-1)/(x-1)^5, {x, 0, 40}], x] (* or *) LinearRecurrence[{5, -10, 10, -5, 1}, {0, 0, 0, 1, 20, 87, 244}, 40] (* Harvey P. Dale, Mar 06 2016 *)

CROSSREFS

Column k=2 of A243608.

Sequence in context: A211158 A154077 A027849 * A219824 A339343 A306987

Adjacent sequences:  A243642 A243643 A243644 * A243646 A243647 A243648

KEYWORD

nonn,easy,changed

AUTHOR

Alois P. Heinz, Jun 08 2014

STATUS

approved

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Last modified October 4 08:15 EDT 2022. Contains 357238 sequences. (Running on oeis4.)