test and testparm for interaction terms

[Eve Wang]

reg y x1 x2 x1#x2

If the coefficient of x1 is significant, the coefficient of x2 is not significant, and the coefficient of x1#x2 is significant. How can I say that there is a joint effect of x1 and x2 on y?

Should I use the following code 1:
test x1=x2= x1#x2=0

or the code 2:
testparm x1#x2

or the code 3:
testparm x1 x2 x1#x2

What are the differences among those three codes?
Thanks a lot!

[Clyde Schechter]

Duplicate post. Asked and answered at https://www.statalist.org/forums/for...eraction-terms.