A <a href="https://en.wikipedia.org/wiki/Monkey_saddle" class="extiw" title="w:Monkey saddle">monkey saddle</a> surface, with the equation <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=-ax^{2}y+by^{3}\ a,b&gt;0}"><semantics><mrow class="MJX-TeXAtom-ORD"><mstyle displaystyle="true" scriptlevel="0"><mi>z</mi><mo>=</mo><mo>−<!-- − --></mo><mi>a</mi><msup><mi>x</mi><mrow class="MJX-TeXAtom-ORD"><mn>2</mn></mrow></msup><mi>y</mi><mo>+</mo><mi>b</mi><msup><mi>y</mi><mrow class="MJX-TeXAtom-ORD"><mn>3</mn></mrow></msup><mtext> </mtext><mi>a</mi><mo>,</mo><mi>b</mi><mo>&gt;</mo><mn>0</mn></mstyle></mrow><annotation encoding="application/x-tex">{\displaystyle z=-ax^{2}y+by^{3}\ a,b&gt;0}</annotation></semantics></math></span><img src="https://wikimedia.org/api/rest_v1/media/math/render/svg/80636f770a01f61e415f684c54a227569e829c33" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.671ex; width:24.919ex; height:3.009ex;" alt="{\displaystyle z=-ax^{2}y+by^{3}\ a,b&gt;0}"></span>.
Object made by Estes Objethos Atelier, photo by <a href="//commons.wikimedia.org/wiki/User:Rodrigo.Argenton" title="User:Rodrigo.Argenton">Rodrigo.Argenton</a>
CC BY-SA 4.0
Matemateca (IME/USP), provided via digitalization project by the volunteer:<a href="//commons.wikimedia.org/wiki/User:Rodrigo.Argenton" title="User:Rodrigo.Argenton">Rodrigo Tetsuo Argenton</a>