title: how to zero with size 3D plot of two variable function
body: <p>I want to draw a two variable function plot using octave. First of all, I'm going to use the meshgrid function to create the x,t axis, and then I'm going to create a u(x,t), and the form I want is a summing form. So I'm going to use the for loof, and I need to reset this function to sum=0. I don't know how to reset it to zero.  I wrote down my code.</p>
  <pre><code> clear;
   [x,t] = meshgrid(0:0.2:pi);                                
   sum=0;
   for n=1:5
     sum+=(1/n^2)*sin(n*pi/2)*exp(-1.6352*n^2*pi^2*t/100^2)*sin(n*pi*x/100)/pi^2;
  
   end
  
  
   surf(x,t,sum)
  
  xlabel('x');
  ylabel('t');
  zlabel('solution');    
  </code></pre>
  <p>It seems strange for initating sum=0 since sum variable is multivariable function with x,t.</p>
  <p>how to plot this?</p>