Assumption in Problem 3.9

[Fromdusktilldawn]

In Problem 3.9, it stated that X^TX is positive definite

Which means it is also invertible

But on page 85, it said that X^TX might not be invertible.

Can someone explain this disparity?

[htlin]

Problem 3.9 asks you to solve a special case (invertible case) of the more general task (on page 85).

Hope this helps.