Python| sympy.factorial2() 方法

借助sympy.factorial2()方法，我们可以找到双阶乘。数字的双阶乘由 -

$n!! = \begin{cases} 1 & n = 0 \\ n(n-2)(n-4) \cdots 1 & n\ \text{positive odd} \\ n(n-2)(n-4) \cdots 2 & n\ \text{positive even} \\ (n+2)!!/(n+2) & n\ \text{negative odd} \end{cases}$

Syntax: factorial2(n)

Parameter:
n – It denotes the number whose double factorial is to be calculated.

Returns: Returns the double factorial of number, i. e., n.

示例 #1：

# import sympy 
from sympy import * 
  
n = 10
print("Value of n = {}".format(n))
   
# Use sympy.factorial2() method 
factorial2_n = factorial2(n)  
      
print("Double factorial of n : {}".format(factorial2_n))  

输出：

Value of n = 10
Double factorial of n : 3840

示例 #2：

# import sympy 
from sympy import * 
  
n = -3
print("Value of n = {}".format(n))
   
# Use sympy.factorial2() method 
factorial2_n = factorial2(n)  
      
print("Double factorial of n : {}".format(factorial2_n))  

输出：

Value of n = -3
Double factorial of n : -1