其它
From $1$ to $23$, how many integers can be chosen at most to ensure that no two of chosen numbers differ by $4$? 
Overseas Competition->Knowledge Point->Combinatorics->Constructing and Proving
$1, 5, 9, 13, 17, 21$  $2, 6, 10, 14, 18, 22$  $3, 7, 11, 15, 19, 23$  $4, 8, 12, 16, 20$ 