Author 
Comment/Response 
Rayne

01/27/07 01:18am
Hi all,
I want to plot the cubic equation f(x) = x^3  16x^2 + 3x + 1 = 0. I had:
f=x^316 x^2+3 x+1 == 0
Plot[f,{x,1,20}]
but I was unable to get the plot. The error message says
Plot::plnr: f is not a machinesize real number at x = 0.999999.
Plot::plnr: f is not a machinesize real number at x = 0.148093.
Plot::plnr: f is not a machinesize real number at x = 0.780985.
General::stop: Further output of Plot::plnr
will be suppressed during this calculation.
How do I get around this problem?
Also, I need to define a set of functions for vector calculus. Two of them are the gradient (equivalent to Del[f] in Mathematica) and the laplacian (equivalent to [CapitalDelta]).
I have:
gradient[f_]:={D[f,x],D[f,y],D[f,z]}
and
laplacian[f_]:=D[f,{x,2}]+D[f,{y,2}]+D[f,{z,2}]
As you can see, this only works if the function f has 3 variables. How do I make it more general, that is, for an arbitrary number of variables?
Also, how do I use the Del and CapitalDel functions in Mathematica? I tried Del[f] when f is 2x^33y^2+4z and only got \[Del](2 x^3  3 y^2 + 4 z) as output.
Thank you.
Regards,
Rayne
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