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Japan mathematical olympiad finals 2006 , problem 4

Source: Japan Mathematical Olympiad Finals 2006 , Problem 4

March 5, 2006
combinatorics proposedcombinatorics

Problem Statement

Let m, nm,\ n be integers such that 2mn2\leq m\leq n and let a, aa,\ a' be integers which are less than or equal to mm and let b, bb,\ b' be integers which are less than or equal to nn such that (a,b)(a b).(a,b)\neq (a'\ b'). Given a town of the rectangular shaped chessboard which is made up of msm's road running north and south which is called Line and nsn's road running west and east which is called Street. Denote the intersection point of the aa th Line from the west and bb th Street from the north by AA, and aa' th Line from the west and bb' th Street from the north by B,B, including the edge for both cases.Find all pair of (m,n,a,b,a,b)(m,n,a,b,a', b') such that by passing through each crossroads of the town exactly one time, you can reach the point BB from the point AA including in the start point and goal one.