MathDB
China MO 2023 P3

Source: China MO 2023 P3

December 29, 2022
combinatorics

Problem Statement

Given positive integer m,nm,n, color the points of the regular (2m+2n)(2m+2n)-gon in black and white, 2m2m in black and 2n2n in white. The coloring distance d(B,C)d(B,C) of two black points B,CB,C is defined as the smaller number of white points in the two paths linking the two black points. The coloring distance d(W,X)d(W,X) of two white points W,XW,X is defined as the smaller number of black points in the two paths linking the two white points. We define the matching of black points B\mathcal{B} : label the 2m2m black points with A1,,Am,B1,,BmA_1,\cdots,A_m,B_1,\cdots,B_m satisfying no AiBiA_iB_i intersects inside the gon. We define the matching of white points W\mathcal{W} : label the 2n2n white points with C1,,Cn,D1,,DnC_1,\cdots,C_n,D_1,\cdots,D_n satisfying no CiDiC_iD_i intersects inside the gon. We define P(B)=i=1md(Ai,Bi),P(W)=j=1nd(Cj,Dj)P(\mathcal{B})=\sum^m_{i=1}d(A_i,B_i), P(\mathcal{W} )=\sum^n_{j=1}d(C_j,D_j) . Prove that: maxBP(B)=maxWP(W)\max_{\mathcal{B}}P(\mathcal{B})=\max_{\mathcal{W}}P(\mathcal{W})