OFFSET
1,2
COMMENTS
The triangle T(n, k) is irregularly shaped: 1 <= k <= A239438(n). First row corresponds to n = 1.
The maximal number of points that can be placed on a triangular grid of side n so that no two of them are adjacent is given by A239438(n).
Row n is the coefficients of the independence polynomial of the triangular grid graph, omitting x^0 coefficients. - Eric W. Weisstein, Nov 11 2016
LINKS
Heinrich Ludwig, Table of n, a(n) for n = 1..136
Stan Wagon, Graph Theory Problems from Hexagonal and Traditional Chess, The College Mathematics Journal, Vol. 45, No. 4, September 2014, pp. 278-287
Eric Weisstein's World of Mathematics, Independence Polynomial
Eric Weisstein's World of Mathematics, Triangular Grid Graph
EXAMPLE
Triangle begins:
1;
3;
6, 6, 1;
10, 27, 21, 1;
15, 75, 151, 114, 27, 1;
21, 165, 615, 1137, 999, 353, 27;
28, 315, 1845, 6100, 11565, 12231, 6715, 1686, 150, 2;
...
There is T(10, 19) = 1 way to place 19 points (X) on a grid of side 10 under to the condition mentioned above:
X
. .
. X .
X . . X
. . X . .
. X . . X .
X . . X . . X
. . X . . X . .
. X . . X . . X .
X . . X . . X . . X
This pattern seems to be the densest packing for all n == 1 (mod 3) and n >= 10.
From Eric W. Weisstein, Nov 11 2016: (Start)
Independence polynomials of the n-triangular grid graphs for n = 1, 2, ...:
1 + 3*x,
1 + 6*x + 6*x^2 + x^3,
1 + 10*x + 27*x^2 + 21*x^3 + x^4,
1 + 15*x + 75*x^2 + 151*x^3 + 114*x^4 + 27*x^5 + x^6,
...
(End)
CROSSREFS
KEYWORD
nonn,tabf
AUTHOR
Heinrich Ludwig, Mar 21 2014
STATUS
approved