theoretical expression of IP3!!

hi every one!
can any one help me to find the real theoretical expression of the third intercept point.
some one told me that it is the third drift of the drain to source current compared to the third drift of the gate to source voltage i.e.

IP3=d3IDS/dVGS3.

if it is true please, is there reference that could help to find it?

thanks in advance!

I think its not true