Question 51
(SWE 4) A subset $S$ of the set of integers $0,\ldots,99$ is said to have property A if it is impossible to fill a crossword puzzle with $2$ rows and $2$ columns with numbers in $S$ ($0$ is written as $00$, $1$ as $01$, and so on). Determine the maximal number of elements in sets $S$ with property A.