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

2012 4 19
2 个回答
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0 个投票

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

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Alexander
Alexander 2012-4-20

1 个投票

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)

更多回答（1 个）

Caio
Caio 2012-4-20

0 个投票

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.

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