# Expand a function in terms of Legendre polynomials

1. Oct 2, 2006

### Logarythmic

Problem:
Suppose we wish to expand a function defined on the interval (a,b) in terms of Legendre polynomials. Show that the transformation u = (2x-a-b)/(b-a) maps the function onto the interval (-1,1).

How do I even start working with this? I haven't got a clue...

2. Oct 2, 2006

### quasar987

I do not speak english in everyday life, and so the words "maps the function onto the interval (-1,1)" seems confusing to me. I would normally interpret them as "u is such that $u\circ f$:(a,b)-->(-1,1)". But that does not seem to make sense, since we do not know the form of f. What does make sense, is to understand it as "u is such that $f\circ u$:(a,b)-->R is the same as f:(-1,1)-->R". In other words, you need to show that $f\circ u$ is a restriction of f. But that is easy, you only need to show that u is onto from (a,b) to (-1,1).

3. Oct 2, 2006

### Logarythmic

Ok, I'm really not good at linear algebra so I think I'll jump this problem. ;)