Question 122
Given A Square Grid $n \times n$ ($n \ge 4$). Initially each of $n$ Cells of One Diagonal Contains a Sign “+” while each other cell contains a sign “-”. by one move it is allowed to choose a row or a column Prove that at any moment the number of signs “+” is not less than $n$. (R. karasev) in a checkered table $n \times n$ ($n \ge 4$) delivered $n$ signs “+” in the cells of one diagonal and “-” signs in all other cells. It is allowed to change all signs to opposite ones in some row or in some column. prove that after any number of such operations, the table will remain at least $n$ pluses. (R. Karaseev)