I am having trouble describing in words the surface created by the function z = 4 - abs(x) - abs(y).

I know exactly what it look likes. It is plane with a high point at (0,0,4), with "creases" along the x = 0 and y = 0.

Can anyone suggest the correct "mathematical defintion" of the surface? I dont think "creases" is exactly the mathematical term they wish me to use.

Many thanks for any help.

Tammy