In the one-dimensional situation the p -superharmonic functions are exactly the concave functions and in several dimensions, the concave functions are p- superharmonic, simultaneously for all p, but there are p-superharmonic functions that it aren't concave. How can we see this?

Find the local maximum and minimum values and saddle points of the function. If you have three-dimensional graphing software, graph the function with a domain and viewpoint that reveal all the important aspects of the function
$f(x,y)=x^3-6xy+8y^3$

$\frac{1}{\sec <!--\; \; -->(x)}$ in derivative?

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