sympy hangs when trying to solve a simple algebraic equation

Question

I recently reinstalled my python environment and a code that used to work very quickly now creeps at best (usually just hangs taking up more and more memory).

The point at which the code hangs is:

solve(exp(-alpha * x**2) - 0.01, alpha)

I've been able to reproduce this problem with a fresh IPython 0.13.1 session:

In [1]: from sympy import solve, Symbol, exp
In [2]: x = 14.7296138519
In [3]: alpha = Symbol('alpha', real=True)
In [4]: solve(exp(-alpha * x**2) - 0.01, alpha)

this works for integers but also quite slow. In the original code I looped over this looking for hundreds of different alpha's for different values of x (other than 14.7296138519) and it didn't take more than a second.

any thoughts?

I have '0.7.1.rc1' and it solves instantly. Perhaps there is a regression — John La Rooy, Jun 13 '13 at 13:08
interesting, I suspected that the reinstallation of my python env had something to do with it. I'll roll back to your version and report in a sec. — Shahar, Jun 13 '13 at 13:13
`easy_install sympy==0.7.1` did it! I've reported this as a bug to sympy's google code. Thanks! — Shahar, Jun 13 '13 at 13:29
Still the same problem in 3.9. [This answer](https://stackoverflow.com/a/17114550/774575) works. — mins, Oct 21 '22 at 08:08

score 5 · Answer 1 · answered Jun 14 '13 at 17:57

5

The rational=False flag was introduced for such cases as this.

>>> q=14.7296138519
>>> solve(exp(-alpha * q**2) - 0.01, alpha, rational=False)
[0.0212257459123917]

(The explanation is given in the issue cited above.)

answered Jun 14 '13 at 17:57

smichr

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Someone should mark this as correct. Based on rep, I'd guess @Shahar won't notice this so maybe a moderator can? – bossylobster Sep 09 '14 at 04:48
This parameter allows solving equations like e^(-at)=k which otherwise hang the kernel in version 3.9. Looks like this answer should be selected. – mins Oct 21 '22 at 08:04

score 2 · Answer 2 · answered Jun 13 '13 at 14:11

2

Rolling back from version 0.7.2 to 0.7.1 solved this problem.

easy_install sympy==0.7.1

I've reported this as a bug to sympy's google code.

answered Jun 13 '13 at 14:11

Shahar

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score 0 · Answer 3 · edited Sep 14 '22 at 12:41

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I am currently running sympy ver: 1.11.1 This is for all I know the latest version. However, hanging as was described, when solving a set of 3 differential for 3 angular double differentials persist.

Eg.:

sols = solve([LE1, LE2, LE3], (the_dd, phi_dd, psi_dd),
             simplify=False, rational=False)

edited Sep 14 '22 at 12:41

lepsch

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answered Sep 09 '22 at 21:33

Frank Koslowski

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sympy hangs when trying to solve a simple algebraic equation

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