@loch Apparently I didn't read this properly at all... Thanks (for some reason I took it to be the max ideal of O_Y,y and it contraction, rather than max ideal of O_X,x and its extension - oh well I better sleep before I make more mistakes)
I guess I expect there to be lots of other etale morphisms, like many smooth schemes such as $\Bbb A^n_K$, but perhaps someone knows one extra condition that makes the separable field extensions the entire class
If $L$ is a finite separable field extension of the field $K$, then $Spec(L)\to Spec(K)$ is etale. Does this actually classify all etale morphisms $Y\to Spec(K)$ to a field? Or are there high dimensional etale covers etc?
Since cohomology of spaces with Z coefficients is just homology of spaces with torsion shifted up a degree, is it true that rational homology and rational cohomology agree in all degrees
Also, there may be (and are) points that aren't critical between a and b, such that f''>0 and f''<0 (i.e. you have to be careful with using your criterion when f'\ne 0)