The term "left factorial" is sometimes used to refer to the subfactorial , the first few values for n = 1, 2, ... are 1, 3, 9, 33, 153, 873, 5913, ... (Sloane's A007489).

Unfortunately, the same term and notation are also applied to the factorial sum

for n = 0, 1, ... are given by 0, 1, 2, 4, 10, 34, 154, 874, ... (Sloane's A003422). The left factorial is always even. is prime for n = 4, 5, 6, 9, 10, 11, 12, 31, 77, ... (Sloane's A100614).

 

Factorial Sums, Subfactorial