Centrosymmetric Solutions to the N+k Queens Problem€¦Centrosymmetric Solutions to the N +k Queens...
Transcript of Centrosymmetric Solutions to the N+k Queens Problem€¦Centrosymmetric Solutions to the N +k Queens...
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Centrosymmetric Solutions tothe N + k Queens Problem
Chatham, Doyle, Jeffers,Kosters, Skaggs, and Ward
Recreational Mathematics CPSMAA MathFestLexington KYAugust 6, 2011
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N Queens Problemn queens on n × n chessboardno two queens are on same row, column, ordiagonal
Figure: Solution to 5 queens problem
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N + k Queens Problemn + k queens, k pawns on n × n chessboardpawn between queens in same row,column, or diagonal
Figure: Solution to 8+2 queens problem
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Symmetries
A solution to an n + k queens problem can becentrosymmetric (symmetric wrt 180-degreerotations, but not 90-degree rotations)
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Symmetries
A solution to an n + k queens problem can bedoubly centrosymmetric (symmetric wrt 90-degreerotations)
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Symmetries
A solution to an n + k queens problem can beordinary (neither centrosymmetric nor doublycentrosymmetric)
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SymmetriesA solution to an n + k queens problem can beordinary (neither centrosymmetric nor doublycentrosymmetric)
No solution (with n > 1) is symmetric withrespect to reflection.
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Centrosymmetric solutions
No such solutions if n even and k odd
If n odd and k even, queen in central squareIf n odd and k odd, pawn in central squareEven number of solutions
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Centrosymmetric solutions
No such solutions if n even and k oddIf n odd and k even, queen in central square
If n odd and k odd, pawn in central squareEven number of solutions
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Centrosymmetric solutions
No such solutions if n even and k oddIf n odd and k even, queen in central squareIf n odd and k odd, pawn in central square
Even number of solutions
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Centrosymmetric solutions
No such solutions if n even and k oddIf n odd and k even, queen in central squareIf n odd and k odd, pawn in central squareEven number of solutions
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Doubly centrosymmetric solutions
Eithern ≡ 0 (mod 4) and k ≡ 0 (mod 4),n ≡ 1 (mod 4) and k ≡ 0 (mod 4), orn ≡ 3 (mod 4) and k ≡ 1 (mod 4)
No doubly centrosymmetric solution ton + 1 queens problem
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Doubly centrosymmetric solutions
Eithern ≡ 0 (mod 4) and k ≡ 0 (mod 4),n ≡ 1 (mod 4) and k ≡ 0 (mod 4), orn ≡ 3 (mod 4) and k ≡ 1 (mod 4)
No doubly centrosymmetric solution ton + 1 queens problem
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Powers of 2 in solution numbers
Proposition
Suppose n = 4s with s ⩾ 2, k = 4t, and n ⩾ 7k.Then the number of doubly centrosymmetricsolutions to the n + k queens problem is divisible by2s−7t.
Figure: Note outer ring of squares
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Powers of 2 in solution numbers
Proposition
Suppose n = 4s with s ⩾ 2, k = 4t, and n ⩾ 7k.Then the number of doubly centrosymmetricsolutions to the n + k queens problem is divisible by2s−7t.
Figure: Note outer ring of squares
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Powers of 2 in solution numbers
Proposition
Suppose n = 4s + 1 with s ⩾ 4, k = 4t, and n ⩾ 7k.Then the number of doubly centrosymmetricsolutions to the n + k queens problem is divisible by2s−7t.
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Powers of 2 in solution numbers
Proposition
Suppose n = 4s + 3 with s ⩾ 3, k = 4t + 1, andn ⩾ 7k. Then the number of doubly centrosymmetricsolutions to the n + k queens problem is divisible by2s−7t−1.
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Different symmetries for diff. pieces
Figure: 14+4 queens solution, queens centrosymmetric,pawns ordinary
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Different symmetries for diff. pieces
Figure: 7+2 queens solution, queens doublycentrosymmetric, pawns centrosymmetric
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Queens with reflective symmetry
Figure: 21+14 queens solution, queens symmetric wrtreflection across central row
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Reflective symmetry for queens
Proposition
Given a solution to the n + k queens problem (withn > 1),
1 the queens are not symmetric with respect toreflection across a diagonal,
2 if n is even, the queens are not symmetric withrespect to vertical or horizontal reflection, and
3 if n = 2s + 1 and the queens are symmetric withrespect to vertical or horizontal reflection, thenk ⩾ s + 1.
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Reflective symmetry for queens
Proposition
Given a solution to the n + k queens problem (withn > 1),
1 the queens are not symmetric with respect toreflection across a diagonal,
2 if n is even, the queens are not symmetric withrespect to vertical or horizontal reflection, and
3 if n = 2s + 1 and the queens are symmetric withrespect to vertical or horizontal reflection, thenk ⩾ s + 1.
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Reflective symmetry for queens
Proposition
Given a solution to the n + k queens problem (withn > 1),
1 the queens are not symmetric with respect toreflection across a diagonal,
2 if n is even, the queens are not symmetric withrespect to vertical or horizontal reflection, and
3 if n = 2s + 1 and the queens are symmetric withrespect to vertical or horizontal reflection, thenk ⩾ s + 1.
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References
N+k Queens Problem Page:npluskqueens.infon-Queens bibliography:www.liacs.nl/home/kosters/nqueens/M. Kraitchik, Mathematical Recreations,2nd ed., Dover Publications Inc., New York,1953.