The grid directly below is a prime square: the numbers in the rows (769, 953, 797) and columns (797, 659, 937) are all prime, none begins with zero, and no two row or column primes are the same. The grid is also "ambidextrous," because the numbers in the rows are still prime if read backward (from right to left). The square is not, however, "omnidextrous." To earn that label, its columns, too, would have to be prime when read backward (bottom to top), and its diagonals would have to be prime in both directions. In this case, 956 is even and therefore not prime.
This puzzle asks you to create other prime squares. To warm up, build a prime 4-square (consisting of four-digit numbers) that uses nine distinct digits. This square need not be ambidextrous. (You can find a table of prime numbers at www.sciam.com/ontheweb.)
This article was originally published with the title Prime Squares.