Factorial is a mathematical term. There is only a calculation method for the factor, and the simple formula can only be calculated hard. For example, if the required number is 4, the factorial formula is1× 2××× 4, and the product obtained is 24, that is, the factorial of 4. For example, if the required number is 6, the factorial formula is 1× 2× 3× 6, and the product is 720, which is the factorial of 6. For example, if the required number is n, the factorial formula is 1× 2× 3×...n, and the product obtained is x, which is the factorial of n. The necessity of the factorial definition: because the factorial of a positive integer is a continuous multiplication operation, the result of multiplying 0 by any real number is 0. So we can't generalize or deduce 0x= 1 with the definition of positive integer factorial. That is, "0x= 1" cannot be explained in the sense of multiplication. The definition of "0x" is only to facilitate the expression and operation of related formulas.