1. Subscribertalzamir
    Art, not a Toil
    60.13N / 25.01E
    Joined
    19 Sep '11
    Moves
    45045
    24 Apr '13 11:07
    If I pick four different squares from a chessboard.. say, D1, F2, E4 and C3.. their center points can (and in this case do) form a square. If four different squares are chosen randomly, how likely is it that they do form a square?
  2. Subscriberjoe shmo
    Strange Egg
    podunk, PA
    Joined
    10 Dec '06
    Moves
    7733
    28 Apr '13 16:28
    Originally posted by talzamir
    If I pick four different squares from a chessboard.. say, D1, F2, E4 and C3.. their center points can (and in this case do) form a square. If four different squares are chosen randomly, how likely is it that they do form a square?
    I'm not much for Combinatorics, but here is what I got

    First the number of distinct ways of selecting 4, boxes out of 64

    C(4,64) = 64!/(4!*60!) = 635376

    Next, looking at each column

    The number of ways of selecting 2, of 8 boxes

    C(2,8) = 8!/(2*6!) = 28

    In order to make a square we need to select 4 points, 2 points in each column.
    Number of ways of doing this

    "Possible" square configurations = 28*7 = 196

    of those 196 combinations, 1/7 are square configurations

    Square configurations = 28

    Probability of selecting a square configuration = 28/635376
  3. Subscriberjoe shmo
    Strange Egg
    podunk, PA
    Joined
    10 Dec '06
    Moves
    7733
    28 Apr '13 17:44
    Originally posted by joe shmo
    I'm not much for Combinatorics, but here is what I got

    First the number of distinct ways of selecting 4, boxes out of 64

    C(4,64) = 64!/(4!*60!) = 635376

    Next, looking at each column

    The number of ways of selecting 2, of 8 boxes

    C(2,8) = 8!/(2*6!) = 28

    In order to make a square we need to select 4 points, 2 points in each column.
    Number of ...[text shortened]...

    Square configurations = 28

    Probability of selecting a square configuration = 28/635376
    Nevermind, pretty sure my answer is wrong. Back to the drawing board!
  4. Standard memberforkedknight
    Defend the Universe
    127.0.0.1
    Joined
    18 Dec '03
    Moves
    16018
    28 Apr '13 17:511 edit
    Originally posted by joe shmo
    Nevermind, pretty sure my answer is wrong. Back to the drawing board!
    Yes, it seems like you're missing all of the diagonal squares that you can make on the board.

    e.g.:
    B1 - A2 - B3 - C2
    B1 - A3 - C4 - D2
    B1 - A4 - D5 - E2
    B1 - A5 - E6 - F2
    etc...
  5. Subscriberjoe shmo
    Strange Egg
    podunk, PA
    Joined
    10 Dec '06
    Moves
    7733
    29 Apr '13 01:56
    Originally posted by forkedknight
    Yes, it seems like you're missing all of the diagonal squares that you can make on the board.

    e.g.:
    B1 - A2 - B3 - C2
    B1 - A3 - C4 - D2
    B1 - A4 - D5 - E2
    B1 - A5 - E6 - F2
    etc...
    I think I missed some other things as well.
  6. Standard memberforkedknight
    Defend the Universe
    127.0.0.1
    Joined
    18 Dec '03
    Moves
    16018
    29 Apr '13 22:392 edits
    I'm trying to do this without writing a program, so I'm determining some limitations first.

    Including reflections and rotations, there are a total of 10 unique starting locations on the chessboard.
    (A1, A2, A3, A4, B2, B3, B4, C3, C4, D4)

    Including reflections and rotations, there are a total of 19 edges that make it possible to form a square on the chessboard.
    (0, 1-7)
    (1, 1-6)
    (2, 2-5)
    (3, 3-4)

    Now, to pare down which of the 19 edges can be used with each of the 10 starting squares, and I should have the number.

    Feel free to correct me if I'm wrong on either of these points.
  7. Subscribertalzamir
    Art, not a Toil
    60.13N / 25.01E
    Joined
    19 Sep '11
    Moves
    45045
    30 Apr '13 05:48
    Sounds like much the same way I was thinking of it, though in a diferent order. I was thinking of starting with with the various shapes, then calculating how many of each there are. I got 24 different kinds of squares on first count, and between 1 and 49 of each type.
  8. Subscribertalzamir
    Art, not a Toil
    60.13N / 25.01E
    Joined
    19 Sep '11
    Moves
    45045
    05 May '13 13:34
    I wonder if this is correct, or if it could be made more elegant.

    Types of squares:
    a1-b1-b2-a2 (1,0 square. from first square to the next, one step to the right, none towards the 8th row)
    ..
    a1-h1-h8-a8. (7,0 square)

    Then there are some in a 45 degree angle.
    b1-c2-b3-a2. (1,1 square).
    ..
    d1-g4-d7-a4 (3,3 square).

    And the rest.
    b1-d2-c4-a3 (2,1 square)

    A square with dimensions (x,y) as defined above are possible as long as x + y < 8, and fit on the board in (8 - (x+y))^2 positions. Also, due to symmetry squares (0,1) and (1,0) are the same square, but (1,2) and (2,1) are not.

    Thus,
    (0,1): 49
    (0,2) and (1,1): 36
    (0,3), (1,2), and (2,1): 25
    (0,4), (1,3), (2,2) and (3,1): 16
    (0,5), (1,4), (2,3), (3,2), (4,1): 9
    (0,6), (1,5), (2,4), (3,3), (4,2), (5,1): 4
    (0,7): 1

    1x49 + 2x36 + 3x25 + 4x16 + 4x9 + 6x4 + 1x1 = 50 + 72 + 124 = 266 different quadruplets that form a square out of a total of 64! / (60!4!) = 635,376 so the probability of randomly picking a foursome that forms a square on the chessboard is about 0.04%.
Back to Top