Question: Why won't this plot ?

 I have the following equation that I want to plot for theta and t with other parameter values in the equation fixed as below. But when I try to animate the solution I get an 'unable to plot' error message. What can I do to plot the equation, U(theta, t) below.

 U(theta, t) := 1/2/Pi+Q*sin(k*theta)+P*sin(m*theta)+1/4*(-I*Q*exp(1/2*k*(-sigma^2*k+2*I*theta))+I*Q*exp(-1/2*k*(sigma^2*k+2*I*theta))-I*P*exp(1/2*m*(-sigma^2*m+2*I*theta))+I*P*exp(-1/2*m*(sigma^2*m+2*I*theta))-2*Q*sin(k*theta)-2*P*sin(m*theta))*t/Pi-1/16*(-2*I*P*exp(1/2*m*(-sigma^2*m+2*I*theta))+2*I*P*exp(-1/2*m*(sigma^2*m+2*I*theta))-2*I*Q*exp(1/2*k*(-sigma^2*k+2*I*theta))+2*I*Q*exp(-1/2*k*(sigma^2*k+2*I*theta))+I*Q*exp(k*(-sigma^2*k+I*theta))+I*P*exp(m*(-sigma^2*m+I*theta))-I*P*exp(-m*(sigma^2*m+I*theta))-I*Q*exp(-k*(sigma^2*k+I*theta))-2*P*sin(m*theta)-2*Q*sin(k*theta))*t^2/Pi^2+1/96*(-I*Q*exp(1/2*k*(-3*sigma^2*k+2*I*theta))+I*Q*exp(-1/2*k*(3*sigma^2*k+2*I*theta))+I*P*exp(-1/2*m*(3*sigma^2*m+2*I*theta))+3*I*Q*exp(-1/2*k*(sigma^2*k+2*I*theta))+3*I*Q*exp(k*(-sigma^2*k+I*theta))-3*I*Q*exp(-k*(sigma^2*k+I*theta))-3*I*Q*exp(1/2*k*(-sigma^2*k+2*I*theta))+3*I*P*exp(-1/2*m*(sigma^2*m+2*I*theta))+3*I*P*exp(m*(-sigma^2*m+I*theta))-3*I*P*exp(1/2*m*(-sigma^2*m+2*I*theta))-3*I*P*exp(-m*(sigma^2*m+I*theta))-I*P*exp(1/2*m*(-3*sigma^2*m+2*I*theta))-2*Q*sin(k*theta)-2*P*sin(m*theta))*t^3/Pi^3:

k:=1;

m:=1;

P:=1;

Q:=1;

sigma:='sigma';

 eval(U(theta,t));

 with(plots):

 animate(plot3d,[U(theta,t),theta=-Pi..Pi,t=0..10],sigma=0..4, view= 0..1,axes=boxed);

 The same happens with the following equation.

 U(theta, t) := 1/2/Pi+Q*sin(k*theta)+P*cos(m*theta)+1/4*(2*P*exp(-1/2*sigma^2*m^2)*cos(m*theta)-I*Q*exp(-1/2*k*(-2*I*theta+k*sigma^2))+I*Q*exp(-1/2*k*(2*I*theta+k*sigma^2))-2*Q*sin(k*theta)-2*P*cos(m*theta))*t/Pi+1/16*(2*exp(-sigma^2*m^2)*P*cos(m*theta)-I*Q*exp((-k*sigma^2+I*theta)*k)-2*I*Q*exp(-1/2*k*(2*I*theta+k*sigma^2))+I*Q*exp(-k*(k*sigma^2+I*theta))+2*I*Q*exp(-1/2*k*(-2*I*theta+k*sigma^2))-4*P*exp(-1/2*sigma^2*m^2)*cos(m*theta)+2*Q*sin(k*theta)+2*P*cos(m*theta))*t^2/Pi^2+1/96*(P*exp(1/2*m*(-sigma^2*m+2*I*theta))+P*exp(-1/2*m*(sigma^2*m+2*I*theta))-3*I*Q*exp(-k*(k*sigma^2+I*theta))-I*Q*exp(1/2*k*(-3*k*sigma^2+2*I*theta))-2*P*exp((-sigma^2*m+I*theta)*m)-2*P*exp(-m*(sigma^2*m+I*theta))+I*Q*exp(-1/2*k*(3*k*sigma^2+2*I*theta))+3*I*Q*exp((-k*sigma^2+I*theta)*k)+P*exp(1/2*m*(-3*sigma^2*m+2*I*theta))+P*exp(-1/2*m*(3*sigma^2*m+2*I*theta))+3*I*exp(-1/2*k*(2*I*theta+k*sigma^2))*Q-3*I*Q*exp(-1/2*k*(-2*I*theta+k*sigma^2))-2*exp(-sigma^2*m^2)*P*cos(m*theta)+4*P*exp(-1/2*sigma^2*m^2)*cos(m*theta)-2*Q*sin(k*theta)-2*P*cos(m*theta))*t^3/Pi^3;

k:=1;

m:=1;

P:=1;

Q:=1;

sigma:='sigma';

 eval(U(theta,t));

 with(plots):

 animate(plot3d,[U(theta,t),theta=-Pi..Pi,t=0..10],sigma=0..4, view= 0..1,axes=boxed);