MathDB

Consider a

25\times25

chessboard with cells

C(i,j)

for

1\le i,j\le25

. Find the smallest possible number

n

of colors with which these cells can be colored subject to the following condition: For

1\le i<j\le25

and for

1\le s<t\le25

, the three cells

C(i,s)

C(j,s)

C(j,t)

carry at least two different colors.
(Proposed by Gerhard Woeginger, Austria)

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