Python| sympy.bell() 方法
借助sympy.bell()方法，我们可以在 SymPy 中找到贝尔数和贝尔多项式。
钟（n） -
Syntax: bell(n)
Parameter:
n – It denotes the order of the bell number.
Returns: Returns the nth bell number.
示例 #1：
# import sympy
from sympy import * n = 5
print("Value of n = {}".format(n))
# Use sympy.bell() method
nth_bell = bell(n)
print("Value of nth bell number : {}".format(nth_bell))
输出：
Value of n = 5
Value of nth bell number : 52
钟（n，k）——
Syntax: bell(n, k)
Parameter:
n – It denotes the order of the bell polynomial.
k – It denotes the variable in the bell polynomial.
Returns: Returns the expression of the bell polynomial or its value.
示例 #2：
# import sympy
from sympy import * n = 5
k = symbols('x')
print("Value of n = {} and k = {}".format(n, k))
# Use sympy.bell() method
nth_bell_poly = bell(n, k)
print("The nth bell polynomial : {}".format(nth_bell_poly))
输出：
Value of n = 5 and k = x
The nth bell polynomial : x**5 + 10*x**4 + 25*x**3 + 15*x**2 + x
示例#3：
# import sympy
from sympy import * n = 5
k = 3
print("Value of n = {} and k = {}".format(n, k))
# Use sympy.bell() method
nth_bell_poly = bell(n, k)
print("The nth bell polynomial value : {}".format(nth_bell_poly))
输出：
Value of n = 5 and k = 3
The nth bell polynomial value : 1866
钟（n，k，（x1，x2，x3，…））——
Syntax: bell(n, k, (x1, x2, x3, …))
Parameter:
n – It denotes the order of the bell polynomial of second kind.
k – It is a parameter in the bell polynomial of second kind.
(x1, x2, x3, …) – It denotes the tuple of variable symbols.
Returns: Returns the Bell polynomials of the second kind.
示例 #4：
# import sympy
from sympy import * n = 5
k = 3
variables = symbols('x:6')[1:]
print("Value of n = {}, k = {} and variables = {}".format(n, k, variables))
# Use sympy.bell() method
nth_bell_poly = bell(n, k, variables)
print("The nth bell polynomial of second kind : {}".format(nth_bell_poly))
输出：
Value of n = 5, k = 3 and variables = (x1, x2, x3, x4, x5)
The nth bell polynomial of second kind : 10*x1**2*x3 + 15*x1*x2**2