SymPy when trying to solve equations, you get an infinite loop

Question

I’m trying to solve the equation

15*exp(ln(0.5)/4 * x) = 1

and I get into an infinity loop

x = symbols("x")

fun = 15 * exp(log(0.5)/4 * x)

eq = Eq(fun, 1)

ans = solve(eq, x) #infinity loop

SymPy version 1.11.1

python version 3.10

Asked By: maximpalshin

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Source

Answer 1

When working with powers raised to a floating exponent, either use nsolve or convert the floating number — especially a simple one like 0.5 — to a Rational:

>>> from sympy import *
>>> from sympy.abc import x
>>> solve(15 * exp(log(S.Half)/4 * x)-1)
[4*log(15)/log(2)]
15.6275623824341

>>> nsolve(15 * exp(log(.5)/4 * x)-1, 1)
15.6275623824341

Answered By: smichr

Answer 2

This is because of the behaviour of solve under the default rational=True setting. Pass rational=False:

In [2]: eq
Out[2]: 
    -0.173286795139986⋅x    
15⋅ℯ                     = 1

In [3]: solve(eq, x, rational=False)
Out[3]: [15.6275623824341]

Answered By: Oscar Benjamin

SymPy when trying to solve equations, you get an infinite loop

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