Thanks. But Maple gives "Plotting error, unknown element _VIEWPOINT" How to fix it? Earlier I tried to do something like this: restart; addcoords(NEW,[v1,v2,v3],[1.*v2,1.*v1,0.5*v3]); p:=plot3d(x*cos(y)^2, x=0..2*Pi, y=0..Pi, axes=normal, color=yellow, coords=NEW): ROT:=proc(plt,pam::anything) plottools[rotate](plt, 0, 0, pam): end proc: plots[animate](ROT,[p,angle,NEW],angle=0..2*Pi); And even wanted to add a new coordinate system for each frame :-) But I could not move the coordinate system with plot...

Anyway, I have accumulated many fast programs written in Fortran, but I like tools for working with graphics in Maple (pointplot, curve, 3D graphics, etc.) Also i need "LinearAlgebra Package". So, my way - to solve difficult problems in one program (numerical calculations and visualization).

Thank you Axel, your example was very helpful. Unfortunately, I can not do it in FORTRAN right now. My problem was solved in a strange way. The fact is that "WATCOM" is working properly, but for some reason I had to replace names of directories containing header files "maplec.h", "mplshlib.h" in "WATCOM" folder (he did not want to work with directory named "maple11", but work with "maple11.abc" or other). Similarly had to rewrite the way in "setvars.bat". I do not know why. But it is not all. After executing Maple-commands the modified value "REPLY_1" (my answer) become inaccessible from Maple (somehow modified values of all input parameters of the procedure were unavailable). So I had to replace "SUBROUTINE" to "FUNCTION" in the Fortran program and return result through name "periodic_integral". I know that to access the modified value, the parameter must be named, like >a:=4: function('a'): a; May be there's another way to do it?