<p>A monkey saddle surface, with the equation<br></p>
<span class="mwe-math-element"><span class="mwe-math-mathml-inline mwe-math-mathml-a11y" style="display: none;"><math xmlns="http://www.w3.org/1998/Math/MathML" alttext="{\displaystyle z=x^{3}-3xy^{2}\,}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mi>z</mi><mo>=</mo><msup><mi>x</mi><mrow class="MJX-TeXAtom-ORD"><mn>3</mn></mrow></msup><mo>−<!-- − --></mo><mn>3</mn><mi>x</mi><msup><mi>y</mi><mrow class="MJX-TeXAtom-ORD"><mn>2</mn></mrow></msup><mspace width="thinmathspace"></mspace></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle z=x^{3}-3xy^{2}\,}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/670facedd0b1c4c73879dbcfe1331562fd163d3f" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.671ex; width:14.505ex; height:3.009ex;" alt="{\displaystyle z=x^{3}-3xy^{2}\,}"></span>
<a href="//commons.wikimedia.org/wiki/User:Inductiveload" title="User:Inductiveload">Inductiveload</a>
Self-made, done in MuPad