Notational confusion of multivariable derivatives

I think there's several different confusions that arise from multivariable derivative notation:

• The thing where ${\frac {\partial w}{\partial t}}$ can mean two different things on LHS and RHS when $t$ is used as both an initial and intermediate variable. (See Folland for details.)
• The thing where if $f(x,y)=(x^{2},y^{2})$ then ${\frac {\partial f}{\partial x}}(x,x)$ feels like it might be $(2x,2x)$ even though it's actually $(2x,0)$. (Example from Tao.) See also [1]
• The ambiguity of expressions like $\nabla f(Ax)$

The derivative as a linear transformation in the several variable case and a number in the single-variable case

• The thing where the total derivative for $n=m=1$ "should" be a function but people treat it as a number. Refer to "Appendix A: Perorations of Dieudonne" (p. 337) in Pugh's Real Mathematical Analysis.

Total derivative versus derivative matrix

Technically the total derivative at a point is a linear transformation, whereas the derivative matrix is a matrix so an array of numbers arranged in a certain order. However, there is a one-to-one correspondence between linear transformations $\mathbf {R} ^{n}\to \mathbf {R} ^{m}$ and $m$ by $n$ matrices, so many books call the total derivative a matrix or equate the two.

A similar confusion exists in the teaching of linear algebra, where sometimes only matrices are mentioned.

See also

• Summary table of multivariable derivatives