multivariable-calculus
Question: Find all critical points of $f(x,y)=x^3+y^4-6x-2y^2$
Apply 2nd Derivative test to each point and determine whether it is local maximum, local minimum or saddle point or that the test fails.
I have found six critical points in total and applying the discriminant equation for 2 variables [ discriminant= (fxx)(fyy) – (fxy)^2 , which is 24x(3y^2 -1) in this question ] , what should be my next step?
Hint: what can you say about the fourth power of any real number?
Saddle point for sure. The Hessian of $f$ is: $$ H_f = \begin{pmatrix} 2x(2x^2-3) y e^{-x^2-y^2} & (2x^2-1)(2y^2-1) e^{-x^2-y^2} \\ (2x^2-1)(2y^2-1) e^{-x^2-y^2} & 2y(2y^2-3) x e^{-x^2-y^2} \end{pmatrix}, $$ thus $$ H_f(0,0) = \begin{pmatrix} 0 & 1 \\ 1 & 0 \end{pmatrix}, $$ and $\mathrm{det}(H_f(0,0)) = -1$ not $1$, which shows $(0,0)$ is a saddle point.
Now to answer your first question.
It is impossible for a smooth function that can have $\mathrm{det}(H_f(x_0,y_0)) > 0$ and $f_{xx}(x_0,y_0) = 0$ at $(x_0,y_0)$.
Let's say if $f_{xx}(x_0,y_0) = 0$, then $$ H_f(x_0,y_0) = \begin{pmatrix} 0 & f_{xy}(x_0,y_0) \\ f_{yx}(x_0,y_0) & f_{yy}(x_0,y_0) \end{pmatrix}. $$ Unless you construct some special smooth functions that bear the property that $f_{xy} \neq f_{yx}$, the determinant $\mathrm{det}(H_f(x_0,y_0)) = - (f_{xy}(x_0,y_0))^2 \leq 0$.
For the second question: when the test is inconclusive,
$f_{xx}$ and $f_{yy}$ have different signs, then we have a saddle point. You can look at the picture to see the geometric meaning of this (though in example they are zero): If our viewponint is somewhere on the $y$-axis and to observe the change in $x$, $f_{xx}>0$ means what we see is a concave up curve near the neighborhood of the point of interest $(x_0,y_0)$; Moving our viewponint to somewhere on the $x$-axis and to observe the change in $y$, $f_{yy}<0$ means what we see is a concave down curve. Then, clearly at $(x_0,y_0)$ we have a saddle.
$f_{xx}$ and $f_{yy}$ have the same signs, then we have a local maximum/minimum.
Best Answer
HINT
We have that for $f(x,y)=x^3+y^4-6x-2y^2$
then evaluate
at each critical point and consider the Hessian determinant at each point.
Refer to