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4x1, 2 3x1, 3 2x1, 4 1x1 pieces

Source: XVI May Olympiad (Olimpiada de Mayo) 2010 L2 P5

September 19, 2022
combinatorics

Problem Statement

You have the following pieces: one 4×14\times 1 rectangle, two 3×13\times 1 rectangles, three 2×12\times 1 rectangles, and four 1×11\times 1 squares. Ariel and Bernardo play the following game on a board of n×nn\times n, where nn is a number that Ariel chooses. In each move, Bernardo receives a piece RR from Ariel. Next, Bernardo analyzes if he can place RR on the board so that it has no points in common with any of the previously placed pieces (not even a common vertex). If there is such a location for RR, Bernardo must choose one of them and place RR. The game stops if it is impossible to place RR in the way explained, and Bernardo wins. Ariel wins only if all 1010 pieces have been placed on the board. a) Suppose Ariel gives Bernardo the pieces in decreasing order of size. What is the smallest n that guarantees Ariel victory? b) For the nn found in a), if Bernardo receives the pieces in increasing order of size, is Ariel guaranteed victory?
Note: Each piece must cover exactly a number of unit squares on the board equal to its own size. The sides of the pieces can coincide with parts of the edge of the board.