@Carl Love 

Okay, thats a shame... and sorry for the double post...

@9colai 

Just an small update. I ran into the same problem in another document, and i solved the problem, by substituting t with another variable. For some reason, it couldn't calculate with a variable, used in other parts of the document...

@Preben Alsholm 

I can't give the whole document sorry... But yes, i see that your example produce the same error. But in my situation, the dsolve method works in another document, so i dont think thats, the kind of error i have (y(t) with Theta(t) ICS). It's just fun that it will solve it with method=laplce, but not with other kind of methods, not even numeric.

But i can complete the project with your help so far, and thats great. Thanks for your time :-)

@Preben Alsholm 

This is wired... I could solve the problem in another document, where the differential equation was copy/pasted to the other document. But when i want to do it on the main document, where the ode is dependend on defined values, i get the error message:

> res := dsolve({ICS, lhs(ode) = f(t)});
print(`output redirected...`); # input placeholder
Error, (in dsolve) found the following equations not depending on the unknowns of the input system: {Theta(0) = (6666666667/10000000000)*Pi, (D(Theta))(0) = 0}

I have problems solving the equation with other methods than laplace, in that specific document, but in others, where i copy/pasted the ode, there is no problem?

Can you tell me where the problem is?

And thanks for the help so far! :-)

@Preben Alsholm 

Nice! I have no idea what you did wit hthe programming. I see the idea with splitting up the homogeneous part and the non-homogeneous part, and adding them together again. But the part with:

for i in F do 
  if match(i=A*sin(a*t),t,'s') then
    sol:=sol+evalf(eval(resS,s))
  elif match(i=A*cos(a*t),t,'s') then
    sol:=sol+evalf(eval(resC,s))
  else
    error "no match"  #Shouldn't happen
  end if
end do;

Is to complex for me.

But it works, and it is way faster! Thanks for the help :-)

@acer 

Thanks for the help, the expand trick did it! :-)

@Preben Alsholm 

The did the trick! Perfect! Thank you very much! :-)

Can you tell me if there is a way to solve a huge differential equation faster than the normal dsolve?
I'm asking, because my differential equation contains a fourier series, and the more accurate i want the series to be, the longer the whole differential equation will be. For example, if i have n=50 in the fourier series the differential equation will be:

diff(Theta(t), t, t)+.93*(diff(Theta(t), t))/sqrt(4*Pi^2+0.)+112.56*Theta(t) = 0.*cos(465.36*t)+0.*sin(465.36*t)+0.*sin(444.21*t)+0.*cos(444.21*t)-0.*sin(423.06*t)+0.*sin(401.90*t)+0.*sin(507.67*t)-0.*sin(486.51*t)+0.*sin(232.68*t)-0.*sin(126.92*t)+0.*cos(63.46*t)+0.*sin(63.46*t)+0.*sin(148.07*t)+0.8e-1*cos(222.10*t)-.71*sin(222.10*t)-0.8e-1*sin(306.72*t)-.10*cos(116.34*t)+0.2e-1*cos(232.68*t)+1.88*sin(31.73*t)+.22*sin(285.56*t)+.54*sin(243.26*t)-1.37*sin(137.49*t)+0.1e-1*cos(148.07*t)+.10*cos(137.49*t)+1.74*sin(74.03*t)-0.7e-1*cos(74.03*t)-0.1e-1*cos(126.92*t)-0.2e-1*cos(296.14*t)-0.3e-1*cos(31.73*t)-1.64*sin(95.19*t)-1.83*sin(52.88*t)-0.9e-1*cos(200.95*t)+1.52*sin(116.34*t)+0.1e-1*cos(105.76*t)-.37*sin(264.41*t)+.88*sin(200.95*t)+0.5e-1*cos(52.88*t)+1.22*sin(158.65*t)-0.1e-1*cos(306.72*t)-.10*cos(158.65*t)+0.2e-1*cos(274.99*t)-0.1e-1*cos(169.22*t)-0.2e-1*cos(253.83*t)-0.1e-1*cos(380.75*t)+0.7e-1*cos(370.17*t)-.24*sin(370.17*t)+0.2e-1*cos(359.60*t)-0.5e-1*cos(349.02*t)+.15*sin(349.02*t)-0.2e-1*cos(338.44*t)-0.5e-1*sin(327.87*t)+0.3e-1*cos(327.87*t)+0.2e-1*cos(317.29*t)+0.*sin(105.76*t)+0.*sin(190.38*t)-0.2e-1*cos(285.56*t)+0.4e-1*cos(264.41*t)-1.05*sin(179.80*t)+.10*cos(179.80*t)-.12*cos(475.94*t)+.39*sin(475.94*t)+.12*cos(454.79*t)-.40*sin(454.79*t)+.39*sin(433.63*t)-.12*cos(433.63*t)-0.1e-1*cos(423.06*t)+.11*cos(412.48*t)-.36*sin(412.48*t)+0.1e-1*cos(401.90*t)-0.9e-1*cos(391.33*t)+.31*sin(391.33*t)-0.1e-1*sin(528.82*t)-0.1e-1*cos(528.82*t)-.11*cos(518.24*t)+.33*sin(518.24*t)+0.1e-1*cos(507.67*t)-0.*sin(211.53*t)-.36*sin(497.09*t)+.12*cos(497.09*t)-0.*sin(42.31*t)-0.1e-1*cos(486.51*t)-0.6e-1*cos(243.26*t)-0.2e-1*cos(211.53*t)+0.9e-1*cos(95.19*t)+0.2e-1*cos(190.38*t)-1.91*sin(10.58*t)+0.1e-1*cos(10.58*t)+0.*cos(21.15*t)+0.*sin(274.99*t)-0.*sin(84.61*t)-0.*cos(84.61*t)+0.*sin(21.15*t)-0.*cos(42.31*t)-0.*sin(296.14*t)-0.*sin(169.22*t)-0.*sin(253.83*t)-0.*sin(380.75*t)+0.*sin(359.60*t)-0.*sin(338.44*t)+0.*sin(317.29*t)

I know, it's a crazy one, but is there a method that can solve this differential equation, or maby a larger one, faster than the normal dsolve?

The initial conditions is:
ICS := Theta(0) = Pi, (D(Theta))(0) = 0

@Preben Alsholm 

It's easy to solve it with simple constant, the problem is that my expression is very big.

Lets forget about the dsolve part, i can solve it with laplace....

Why won't Maple plot my expression, when it gets too big?

I want to plot this expression:

-0.*sin(74.03*t)-0.*cos(52.88*t)-0.*sin(31.73*t)+0.*cos(31.73*t)+0.*cos(116.34*t)-0.*sin(116.34*t)+0.*sin(52.88*t)+0.*cos(74.03*t)+0.*sin(95.19*t)+0.*sin(137.49*t)-0.*cos(137.49*t)+5.45*10^(-8)*sin(84.61*t)-5.55*10^(-8)*sin(105.76*t)+5.72*10^(-8)*sin(126.92*t)+6.40*10^(-7)*cos(84.61*t)-6.11*10^(-7)*cos(105.76*t)-5.61*10^(-8)*sin(63.46*t)+5.79*10^(-7)*cos(126.92*t)-5.44*10^(-7)*cos(148.07*t)+6.67*10^(-8)*sin(42.31*t)-5.88*10^(-8)*sin(148.07*t)-6.69*10^(-7)*cos(63.46*t)-1.33*10^(-7)*sin(21.15*t)+7.09*10^(-7)*cos(42.31*t)-0.*cos(95.19*t)+1.02*cos(10.58*t)-.45*sin(10.58*t)-9.01*10^(-7)*cos(21.15*t)+4.79*10^(-699)*(4.43*10^698*cos(10.61*t)+9.80*10^697*sin(10.61*t))*exp(-0.7e-1*t)

But when i plot it with the normal function plot("expression",t=0..60), i get an empty graph?

Thanks Carl Love!

And sorry for making a question i could search for, i did try with google without any luck.

Thanks! :-) need to type this one some times before i will remember that command. But it works!

Thanks! :-) need to type this one some times before i will remember that command. But it works!

Thats it, the first answer. Thanks! :-)

Thats it, the first answer. Thanks! :-)

Thanks! :-)

Thanks! :-)