CODEWITHSUNDEEP
pythonpython-3.xsympy
Dio
Dio

Reputation: 241

Inverting a function in the positive reals

So I am trying to take the inverse of a function which I will then differentiate. I am only interested in solutions in the positive real domain. There is a subproblem here which is how to treat exponents as some examples:

If I put in x^2 = u, I want it to give me u^(1/2).

If I give it the u=logx, I want it to give me the exponential of u. Etc, etc,

Is there a simple way to do this? The problem is that it returns too many solutions, is there a way to just drop the negative solutions? 

from sympy import *
x, b, a, u, t, dt, dW = symbols('x b a u t dt dW', real = True)
utility = Eq(x**2, u)
invutility = solveset(utility, x)

Which gives:

{-sqrt(u), sqrt(u)}

I am only interested in the positive solution.

Upvotes: 0

Views: 130

Answers (1)

smichr
smichr

Reputation: 19029

solve will give you these solutions if you declare the variables as positive:

>>> x, u = var('x u',positive=True)
>>> utility = Eq(x**2, u)
>>> solve(utility,x)
[sqrt(u)]
>>> solve(u-log(x),x)
[exp(u)]

Upvotes: 1

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