How to solve equation using math symbolic and get erfinv as output?

Hi,

When I type this code:

syms x y
solve(erf(x) - y,x)

I get the following

ans =

[ empty sym ]

When it's clear that the answer is x = erfinv(y), if we assume that y belongs to (0,1).

Is there some way in which I can get erfinv(y)as an answer? I guess the problem is that I didn't specify that y is between 0 and 1. If so, how can I do that?

Thanks,

Caio

Answer 1

[Alexander]

Hi Caio,

If you have MATLAB R2012a you can simpliy use the assume command to say that y belongs to (0, 1):

 assume(0<y<1)

 >> solve(erf(x) - y,x)

 ans =

 erfinv(y)

If you have an older version, you need to use evalin:

 evalin(symengine, 'assume(0<y<1)')

You can also try to use the option IgnoreAnalyticConstraints. With this option, the symbolic engine is a little less restrictive when doing calculations:

 syms y clear
 solve(erf(x) - y,x, 'IgnoreAnalyticConstraints', true)

 ans =

 erfinv(y)

Answer 2

[Caio]

Thanks! It keeps not working here, but I'll have a closer on this. If it worked on yours matlab it should work on mine. I have the R2011a.