# Change of variable in an integration

How to indicate a change of variable to Sage in an integration when Sage seems clueless?

Change of variable in an integration

How to indicate a change of variable to Sage in an integration when Sage seems clueless?

add a comment

3

Here is an example you can look at.

http://www.sagenb.org/home/pub/1927/

Edit: If you need to preload the function use the following link

http://ask.sagemath.org/question/1072...

Edit2:

Sorry for the delay. Here is the full solution to your question

```
def changevar(f, eqn, newvar):
dx = diff(eqn.rhs(),newvar)
return f.substitute(eqn)*dx
var('u')
assume(x>0)
xmin=0
xmax=2
eqn=sqrt(u-1)
limits(x)=solve(eqn==x,u)[0].rhs()
limits(2)
ans=changevar(x*cos(x^2+1), x==eqn, u)
integrate(ans,(u,limits(xmin),limits(xmax))).n()
```

That's nice, thanks. But two more questions about that: 1. How to store and load such python functions for further use in other sessions? 2. If we compute a definite integral, how to add the bounds ?

1

Based on Shashank comment, but see extra questions below:

```
def changevar_definite(f, eqn, newvar, oldvar, a, b):
old_assumptions= assumptions() # Keep current assumptions for later restore
assume(oldvar > a, oldvar < b) # This is bad (Maxima pb), see below
h(oldvar)= solve(eqn, newvar)[0].rhs()
g(newvar)= solve(eqn, oldvar)[0].rhs()
dx = diff(g, newvar)
forget()
assume(old_assumptions) # Unfortunately, extra assumptions don't stay local
return f.substitute(oldvar==g(newvar))*dx, h(a), h(b)
```

Okay, let's try it

```
integrand=changevar_definite(x*cos(x^2+1), u==x^2+1, u, x, 0, 2)
integrand # Output: (u |--> 1/2*cos(u), 1, 5)
```

Eventually

```
integral(integrand[0](u), u, integrand[1], integrand[2])# -1/2*sin(1) + 1/2*sin(5)
```

But the following also works:

```
changevar_definite(x*cos(x^2+1), x==sqrt(u-1), u, x, 0, 2)
```

Now, the latter call breaks if I have `assume(oldvar >= a, oldvar <= b)`

instead as I had earlier, because Maxima asks about x being positive or zero. In fact, the assumptions mechanism is too sticky, what I change locally has global side effect. A worse problem is I can't force a more stringent assumption locally (say going from x >= 0 to x > 0).

1- How to efficiently handle those assumptions?

2- How to pass extra-assumptions as arguments to my `changevar_definite`

function?

Please start posting anonymously - your entry will be published after you log in or create a new account.

Asked: ** 2012-01-28 18:59:27 +0100 **

Seen: **1,223 times**

Last updated: **Feb 13 '12**

Substitution of a list of variables

Substitute variable in expression

automatic substitution within functions?

Is there any way I can substitute a combination of variables.

Is applying a ring homomorphism faster than symbolic substitution?

substituting expressions for numbers

substitute expression instead of formal function symbol

How to magically define variables and use functional notation instead of methods

Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.