Can anyone explain the rationale behind plot command, which generates the same graph differently for an identical function?

> restart;
> with(plots); with(plottools);
m := 4;
k := 20;
omega[n] := sqrt(k/m); 
T[n] := evalf(2*Pi/omega[n]); 
td := 0.5*T[n]; 
ratio := evalf(td/T[n]);
p3 := 4*sin(Pi*t/td);
dur := evalf(2*Pi);

enc := u(0) = 0, D(u)(0) = 0; 
eq := m*(diff(u(t), t, t))+k*u(t) = p3; 
sol := unapply(rhs(dsolve([eq, enc])), t); 

Does anyone know what the beneath error stated after the following coding means, can't find
the solution,     
Although looked in mapleprimes site, and thanks for Joe Riel who pointed out the same problem in 
2010,2 June...

But, it seems there is still no vivid solution for such problem, how to follow


k:=20:
m:=4:

p[0] := 1:

tr := 4:

frame:=100:
omega[n]:=sqrt(k/m):

T[n]:=(2 pi/omega[n]):

Can't find what the matter in recursive occurance of this problem. Anyone knows how to sort it out, thanks

k:=20:
m:=4:

p[0] := 1:

tr := 4:

omega[n]:=sqrt(k/m):

T[n]:=(2 pi/omega[n]):

> maxpoint := proc (x) 
local tr, p1;

tr := evalf(x*T[n]):
p1 := piecewise(t1 <= tr, p[0]*t1/tr, p[0]):
maximize(int(p1*sin(omega[n]*(t-t1))/(m*omega[n]), t1 = 0 .. t), t = 0 .. dur) :

Can someone help me to generate the same, identical grpah over several periods in x-axis? In exact term, I mean that I need to plot a periodical grpah, that repeats itself over the interval? It is like given a periodical function and now have to draw it many times...(function can be any form, not only sine and cosine)