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A152125 On a 4 X 4 square grid, there are 14 lattice squares parallel to the axes. What is the fewest dots you can remove from the grid such that at least one vertex of each of the 14 squares is removed? The answer is a(4) = 4. In general a(n) is the answer for an n X n grid. +0
1
0, 1, 2, 4, 8, 12, 17 (list; graph; listen)
OFFSET

1,3

COMMENT

Comment from Robert Israel: This is a "set covering problem", which can be done by integer linear programming for small n.

EXAMPLE

1 X 1: 0 dots, since there are already no squares,

2 X 2: 1 dot, any vertex will do,

3 X 3: 2 dots, the center kills all the small squares and you need one corner to kill the big square,

a(4) = 4: there are 4 disjoint squares, so it must be at least 4, and with a little more work you can find a set of 4 dots that work.

From Robert Israel:

For the 5 X 5 case, Maple confirms that the optimal solution is 8 dots,

which can be placed at

[1, 1], [1, 3], [2, 2], [2, 3], [3, 0], [3, 1], [3, 2], [4, 4]

For the 6 X 6 case, Maple tells me that the optimal solution is 12 dots,

which can be placed at

[0, 5], [1, 1], [1, 2], [1, 4], [2, 0], [2, 3], [2, 4], [3, 1], [3, 3],

[4, 0], [4, 4], [5, 2]

For the 7 X 7 case, Maple tells me that the optimal solution is 17 dots,

which can be placed at

[0, 0], [1, 2], [1, 3], [1, 5], [2, 1], [2, 4], [2, 5], [3, 2], [3, 3],

[3, 4], [4, 1], [4, 2], [4, 5], [5, 1], [5, 3], [5, 4], [6, 6]

CROSSREFS

Sequence in context: A152768 A160686 A046843 this_sequence A100057 A007590 A080476

Adjacent sequences: A152122 A152123 A152124 this_sequence A152126 A152127 A152128

KEYWORD

nonn,more

AUTHOR

Joshua Zucker (joshua.zucker at gmail.com,). Mar 25 2009

EXTENSIONS

a(5)-a(7) from Robert Israel (israel at math.ubc.ca), Mar 25 2009

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Last modified November 25 20:09 EST 2009. Contains 167514 sequences.


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