Smart subs

[Paul Royik]

Is there a function that will convert sqrt(x+1)+x+3 into y^2+y+2 with y=sqrt(x+1)?

Subs is smart enough to make (x**4+x**2+2).subs(x**2, y) into y**2+y+2.

But it doesn't work with initial example. Even sqrt(x+1)+x+1 is not converted into y^2+y

[Francesco Bonazzi]

In [1]: ex = sqrt(x+1)+x+3

In [2]: ex.subs(x, solve(y-sqrt(x+1), x)[0])
Out[2]:
        ____    
 2     ╱  2    
y  + ╲╱  y   + 2

If y is declared to be positive, you get the right expression.

[Paul Royik]

Another smart solution. Thanks a ton.

Maybe you can tell me why the following code doesn't work: https://github.com/sympy/sympy/issues/9233

[Paul Royik]

Your aaproach fails when we want to substitute sqrt(xy+1) for u.

What to solve here?

[Paul Royik]

Is there any way subs can substitute 3x+3y into 3t without factoring first?

I enter 3*(x+y) but sympy automatically converts it into 3x+3y and subs doesn't work.

On Thursday, April 2, 2015 at 12:15:02 PM UTC+3, Francesco Bonazzi wrote:

[Francesco Bonazzi]

Maybe you should read the documentation. Have a look at unevaluated expressions.

[Paul Royik]

OK. Thank you.