Conditionals ("If") and definitions for 3D plots

*To*: mathgroup at smc.vnet.net*Subject*: [mg14094] Conditionals ("If") and definitions for 3D plots*From*: "Julio Vera" <lvera at varela.reu.edu.uy>*Date*: Fri, 25 Sep 1998 03:15:20 -0400*Sender*: owner-wri-mathgroup at wolfram.com

Hello. This is my first posting to the list. I'm very new to Mathematica. I have studied quite a bit of The Mathematica Book in the last 2 weeks, though. A few questions. 1) How to give NO value to the "else" parameter of the function If[expr, then, else] I want a function to be defined ONLY if the condition is true. I tried to leave a blank, a space, and to put the "not exist" symbol, with no luck. I tried the "Which" function too. 2) How to define a dominium range (for a function to be plotted) by means of an equation Instead of giving a maximum and a minimum value to the variables involved (by expressions like {x,-1,1},{y,-1,1}), use a function, maybe an implicit one like 4x^2+y^2==1 . 3) ImplicitPlot3D For Plot, we have Plot3D. Same thing with ParametricPlot and ParametricPlot3D. But I cannot find an equivalent tridimensional function to ImplicitPlot. I have used the normal Mathematica functions, and those in the Graphics' package. Thanks for your help and your time. Julio Vera