Non-linear transformation examples in section about invertibility of linear transformations

[bsoo920]

In 3.5.4 Invertible linear transformations, several non-linear transformations (e.g. f(x) = x^3) were used to illustrate non-invertible transformations. It seems to me that since they are not linear to begin with, it's not very educational to use them in a section on invertible linear transformations. (A transformation that's non-linear is not, by definition, an invertible linear transformation.)

At the very least, the section should mention they are not linear to begin with, but are used anyway to illustrate the invertibility aspect alone.