product of numeric variables in lm

["Harald Hoogstrate" <Hoogstrate@POINTLOGIC.NL>]

Dear all,

 

I am doing a regression with lots of numerical variables on S+ 6.1.

Now I want to include the product of two variables a and b in my regression:

 

lm(y~c+a*b,data="">

 

and I find that the result is very different from what I get if I define D$ab<-D$a*D$b and try

 

lm(y~c+ab,data="">

 

So my questions: what does * do and, more importantly, how can I include the product of two variables without first defining them in my dataframe.

 

Thanks for any help,

 

Harald