The smallest number expressible as the sum of a prime square, prime cube, and prime fourth power is $28$. In fact, there are exactly four numbers below...
By counting carefully it can be seen that a rectangular grid measuring $3$ by $2$ contains eighteen rectangles: Although there exists no rectangular grid that contains exactly...
NOTE: This problem is a significantly more challenging version of Problem 81. In the $5$ by $5$ matrix below, the minimal path sum from the top left...