On a board of n² squares, two queens, two rooks, two bishops, or two knights can always be placed, irrespective of attack or not, in ½(n^{4} - n²) ways. The following formulæ will show in how many of these ways the two pieces may be placed with attack and without:--
With Attack. Without Attack.
2 Queens 5n³ - 6n² + n 3n^{4} - 10n³ + 9n² - 2n ------------------- ------------------------------ 3 6
2 Rooks n³ - n² n^{4} - 2n³ + n² ---------------------- 2
2 Bishops 4n³ - 6n² + 2n 3n^{4} - 4n³ + 3n² - 2n -------------------- ----------------------------- 6 6
2 Knights 4n² - 12n + 8 n^{4} - 9n² + 24n -------------------- 2
(See No. 318, " Lion Hunting.")
