solve equation with double sum

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Hi!

Please help me with my porblem. I have two no-linear equations:

1) f(x)==h(x)

2) g(x)+S_{i,j,k}(x) == 0

I know I can solve (numerically) eq.(1) doing:

x=var('x')
find_root(f(x)==h(x),x,x_min,x_max)

In eq.(2) S_{i,j,k}(x) is a triple sum function of 'x' and i,j and k are the index of the sum.

How can I solve (numerically) eq.(2)?

Waiting for your answers. Thanks a lot! Best regards

Update:

If I run the next code:

import sympy.mpmath

N=20
A=0.7 
G_0 = 37.7
B = 0.36

x = sympy.symbols('x')

def S(x_): 
     return sympy.mpmath.nsum(lambda i, j, k: (12*A**4*x_**6*i**4-30*A**2*x_**3*i**2*(j**2+k**2)+3*(j**2+k**2)**2)/(2*(A**2*x_**3*i**2+j**2+k**2)**(7/2)),[1,N],[1,N],[1,N])

def F(x_):
     return G_0 * (x_ - 1/(x_**2))

print(sympy.mpmath.findroot(F(x) + B*A*sqrt(x)*S(x), [0.85,1]) )

I get the next error:

TypeError: unsupported operand parent(s) for '*': 'Symbolic Ring' and '<class 'sympy.mpmath.ctx_mp_python.mpf'>'

What am I doing wrong?

Best regards!