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Dr. Tar Ortiz wrote: > > We want to plot the function x=4 , but we can not do that. Do you know > how to do it, help us please. > -- > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > Dr. Tar Ortiz Peralta Instituto Tecnológico y de > Estudios Superiores > Profesor Titular de Monterrey > Campus - Toluca > Dir. del Depto. de Ciencias Básicas Cien mts. al nte. de San > Antonio Buenavista > Tel(72)741174,741128 ext.2175 Toluca Edo. de México C. P. > 50252 > Fax(72)741178 > Internet:jortiz at campus.tol.itesm.mx > http:/www.tol.itesm.mx/~amercado/dic/deptos/deptos.html > +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ First off, x=4 is not a function. Functions, by definition are single valued with respect to their argument. If you want to plot a vertical line at x=4 try making a function like: f[x_]:=If[x==4,1000,0]. This may not work well on some plots because it has an infinitely small non-zero range, so you may want to try something like: f[x_]:=If[(x>4-delta&&x<r+delta),1000,0]. To better define this kind of function, look up the theory of the dirac delta function. You can also try plotting: f[x_]:=10^6 Exp[-10^6 (x-4)^2], which is an approximation to a dirac delta function.