Hi there. I've got some doubts about the maxima and minima on this function: f(x,y)=x \sin y. I've looked for critical points, and theres only one at (0,0). The thing is that when I've evaluate the second derivatives I've found that f_{xx}=0, then I have not a defined criteria on this case, cause the criteria that I have defines the concavity for f_{xx}>0 and f_{xx}<0. I don't know what happens when f_{xx}=0. I think the criteria fails, but I don't actually know what to do.

Bye there!