CSE Blog - quant, math, computer science puzzles

Source: AUSTMS Gazette 35 Related Problem: Pebble Placement Puzzle 1 Problem: Peggy aims to place pebbles on an n × n chessboard in the following way. She must place each pebble at the center of a square and no two pebbles can be in the same square. To keep it interesting, Peggy makes sure that no four pebbles form a non-degenerate parallelogram. What is the maximum number of pebbles Peggy can place on the chessboard?

Source: AUSTMS Gazette 35 Problem: There are several pebbles placed on an n × n chessboard, such that each pebble is inside a square and no two pebbles share the same square. Perry decides to play the following game. At each turn, he moves one of the pebbles to an empty neighboring square. After a while, Perry notices that every pebble has passed through every square of the chessboard exactly once and has come back to its original position. Prove that there was a moment when no pebble was on its original position.