The congruum problem is to find a square number x ² such that, when a given number h is added or subtracted, new square numbers are obtained, so that x ² + h = a ² and x ² – h = b ². This problem was posed by the mathematicians Théodore and Jean de Palerma in a mathematical tournament organized by Frederick II in Pisa in 1225. The solution is x = m ² + n ² and h = 4mn(m ² – n ²), where m and n are integers.