LM with summation function

Hi all,

I'm trying to model some data where the y is defined by

y = summation[1 to 50] B1 * x + B2 * x^2 + B3 * x^3

Hopefully that reads clearly for email.

Anyway, if it wasn't for the summation, I know I would do it like this

lm(y ~ x + x2 + x3)

Where x2 and x3 are x^2 and x^3.

However, since each value of x is related to the previous values of  
x, I
don't know how to do this.  Any help is greatly appreciated.