[R] Curve fitting

[Nagu]

Hi,

Problem statement:
 There are two variables Y, X. Y is a response from X. I want to find
a closed form formula to express Y in terms of X.

Comments:
I tried with simple polynomial basis but its not a good fit due to the
non-linear nature of Y.

Are there any set of functions or algorithm to estimate this closed
form transformation? It could involve any trug, algebraic functions
etc...

Thank You,
Nagu