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  2. Chessboard - Wikipedia

    en.wikipedia.org/wiki/Chessboard

    Each square on the board is identified by a unique coordinate pairing, from a1 to h8. [10] In the older descriptive notation, the files are labelled by the piece originally occupying its first rank (e.g. queen, king's rook, queen's bishop), and ranks by the numbers 1 to 8 from each player's point of view, depending on the move being described ...

  3. MacMahon Squares - Wikipedia

    en.wikipedia.org/wiki/MacMahon_Squares

    The goal is to arrange the squares into a 4 by 6 grid so that when two squares share an edge, the common edge is the same color in both squares. In 1964, a supercomputer was used to produce 12,261 solutions to the basic version of the MacMahon Squares puzzle, with a runtime of about 40 hours.

  4. Galileo's paradox - Wikipedia

    en.wikipedia.org/wiki/Galileo's_paradox

    First, a square is an integer which is the square of an integer. Some numbers are squares, while others are not; therefore, all the numbers, including both squares and non-squares, must be more numerous than just the squares. And yet, for every number there is exactly one square; hence, there cannot be more of one than of the other.

  5. Graph coloring - Wikipedia

    en.wikipedia.org/wiki/Graph_coloring

    With only two colors, it cannot be colored at all. With four colors, it can be colored in 24 + 4 × 12 = 72 ways: using all four colors, there are 4! = 24 valid colorings (every assignment of four colors to any 4-vertex graph is a proper coloring); and for every choice of three of the four colors, there are 12 valid 3-colorings. So, for the ...

  6. Mutilated chessboard problem - Wikipedia

    en.wikipedia.org/wiki/Mutilated_chessboard_problem

    The puzzle is impossible to complete. A domino placed on the chessboard will always cover one white square and one black square. Therefore, any collection of dominoes placed on the board will cover equal numbers of squares of each color. But any two opposite squares have the same color: both black or both white.

  7. Wallpaper group - Wikipedia

    en.wikipedia.org/wiki/Wallpaper_group

    Orbifold signature: 4 *2; Coxeter notation: [4 +,4] Lattice: square; Point group: D 4; The group p4g has two centres of rotation of order four (90°), which are each other's mirror image, but it has reflections in only two directions, which are perpendicular. There are rotations of order two (180°) whose centres are located at the ...

  8. Nonogram - Wikipedia

    en.wikipedia.org/wiki/Nonogram

    Using this technique for all rows and columns at the start of the puzzle produces a good head start into completing it. Note: Some rows/columns won't yield any results initially. For example, a row of 20 cells with a clue of 1 4 2 5 will yield 1 + 1 + 4 + 1 + 2 + 1 + 5 = 15. 20 - 15 = 5. None of the clues are greater than 5.

  9. Hadwiger–Nelson problem - Wikipedia

    en.wikipedia.org/wiki/Hadwiger–Nelson_problem

    In geometric graph theory, the Hadwiger–Nelson problem, named after Hugo Hadwiger and Edward Nelson, asks for the minimum number of colors required to color the plane such that no two points at distance 1 from each other have the same color. The answer is unknown, but has been narrowed down to one of the numbers 5, 6 or 7.