substitution with a condition

[Philippe Piot]

Dear All, 

  I cannot figure out how to impose a condition. I have a function that is defined as a piecewise function. When it returns its results with several Min(x,y). I would like to substitute Min(x,X) with "x" since x<X. I have tried something like (here simplified as my function output as several terms). 

with assuming (x<X):
    print (Min(x,X))

but it does not work [i.e. it does not simplify Min(x,X) with x]. Thanks for any suggestions. 

  Best,  -- Philippe. 

[Oscar Benjamin]

Hi Philippe,

There isn't any way to do this properly right now.

We can't use inequalities like `x < X` but we can say that X = x+delta
where delta is positive:
```
In [3]: x, X = symbols('x X', real=True)

In [4]: e = Min(x, X)

In [5]: e
Out[5]: Min(X, x)

In [6]: delta = Symbol('delta', positive=True)

In [7]: e.subs(X, x+delta)
Out[7]: x
```

There is also the direct approach:
```
In [8]: e.subs(Min(x, X), x)
Out[8]: x
```

--
Oscar

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