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$f(x,y)=x^2-y^2$ is your friendly neighbourhood hyperbolic paraboloid. $f(x, y) = |x| - |y|$ naturally has similar appearance. Do shapes of the latter form have a name?

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    You could call $z=|x|+|y|$ a "circular cone" with *circle* here defined under the [1-norm](http://en.wikipedia.org/wiki/Lp_space) $\|\cdot\|_1$ instead of the usual 2-norm $\|\cdot\|_2$, but I can't think of anything standard for $z=|x|-|y|$. I submit the term "four-plane saddle" for any shape given by $z=ax+b|x|+cy+d|y|,$ or any invertible affine transformation thereof, with $\mathrm{sgn}(a+b)=\mathrm{sgn}(-a+b)=\pm1,$ $\mathrm{sgn}(c+d)=\mathrm{sgn}(-c+d)=\mp1.$2011-09-13

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What about hyperbolic superparaboloid analogous to quadric - superquadric?

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    I'll go with it. Mine's just $f(x, y) = |x|^m - |y|^n$ for $m = n = 1$.2011-09-18