ODES: ValueError: setting an array element with a sequence

Question

I am trying to define the following system of ODES in python:

this way:

def rhs(t, P):
    dP = np.zeros_like(P)
    dP[0] = np.sqrt((1 - 3 / P) * (2 + 4 / P**2))
    dP[1] = 1 / math.pow(P,2)
    return dP

However, I am getting an error:

ValueError: setting an array element with a sequence.

I am not sure what is the problem... Would be grateful for any help with this!

Answer 1

Obviously, your state P has 2 components. Thus also the expressions you compute from P will have two components. Then you try to assign these tuples to a single cell in the array dP, which is not possible and results in that error message.

You might want to replace P with its first element P[0] in these expressions. Or use

def rhs(t, u):
    P,phi = u
    dP = ...
    dphi = ...
    return [dP, dphi]