python - sympy and mpmath give "TypeError: cannot create mpf" when using the erf() function within solveset() -

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i have 4 input variables (floats):

  • xmax
  • xmin
  • percentage
  • mode

and want solve following (rather long) equation s:

> (1/2+1/2*erf((log(xmax)-(log(mode)+s**2))/(sqrt(2)*s))-(1/2+1/2*erf((log(xmin)-(log(mode)+s**2))/(sqrt(2)*s))) - percentage == 0

i want use mpmath , sympy solve equation, gives me following error message:

typeerror: cannot create mpf 0.707106781186547*(-s**2 - 0.287682072451781)/s

my code follows:

from mpmath import erf, log, sqrt sympy import symbol, solveset, s  percentage = 0.95 mode = 2 xmin = 1. xmax = 1.5 s = symbol('s')  eqn = (1/2+1/2*erf((log(xmax)-(log(mode)+s**2))/(sqrt(2)*s))-(1/2+1/2*erf((log(xmin)-(log(mode)+s**2))/(sqrt(2)*s))) - percentage)  solveset(eqn, s, domain=s.reals)

mpf float type created mpmath.

i think narrowed down problem erf() function, returns 

emptyset()

when run 

solveset(log(xmax) - (log(mode) + s ** 2), s, domain=s.reals)

i cannot figure out try next, appreciated!

i thought issue math equation solved in matlab, problem coming sympy or mpmath.

losing mpmath import of erf, , use sympy version resolves error.

from sympy import symbol, solveset, s, erf, log, sqrt  percentage = 0.95 mode = 2 xmin = 1. xmax = 1.5 s = symbol('s', real=true)  eqn = (1/2+1/2*erf((log(xmax)-(log(mode)+s**2))/(sqrt(2)*s))-(1/2+1/2*erf((log(xmin)-(log(mode)+s**2))/(sqrt(2)*s))) - percentage)  solveset(eqn, s)

note also:

  • you don't have import log , sqrt mpmath. won't make difference result here them sympy
  • you can specify real domain on variable s, saves doing on solveset call(s).

example of further usage in package tests here if need it.


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