x,i,n = symbols("x i n")
summation(x,(i,1,n))
How do I make x indexed by i?
Without a numerical upper limit, it won't do anything, but otherwise you can use a function-like expression or an indexed variable:
>>> Sum(Indexed('x',i),(i,1,3))
Sum(x[i], (i, 1, 3))
>>> _.doit()
x[1] + x[2] + x[3]
>>> x = Function('x')
>>> Sum(x(i),(i,1,3)).doit()
x(1) + x(2) + x(3)
>>> Sum(x(i),(i,1,n)).doit()
Sum(x(i), (i, 1, n))
来源：https://stackoverflow.com/questions/24834447/how-to-create-an-indexed-variable-in-sympy