load heatproblem
whos
  Name          Size                Bytes  Class     Attributes

  A         10000x10000            873608  double    sparse    
  b         10000x1                 80000  double              

spy(A)
B = reshape(b,100,100);
surfc(B)
t = A \ b;
T = reshape(t,100,100);
surfc(T)