Using sign function in Matlab

Hi, I try to run the following in MATLAB:

  [D,x] = cheb(N);
  D2 = D^2;
   D = D(2:N,2:N); D2 = D2(2:N,2:N);  x = x(2:N);
  A = -1*(sign(x))*D2 + (diag(x))^2;

  
  but I get the following error:
  
  Error using  * 
Inner matrix dimensions must agree.

Error in airy_demo (line 32)
  A = -1*(sign(x))*D2 + (diag(x))^2;

Error in switch_demo (line 52)
      mtx = feval(routine,N);

Error in eigtool_switch_fn (line 383)
    ps_data = switch_demo(fig,ps_data,the_demo);
     
Error while evaluating Menu Callback.
    
 

 This happens only when I use the sign function. Is it given incorrectly? Thanks

Answer 1

[Tommy]

sign(x) returns a matrix the same size as x. If the dimensions of x (and therefore sign(x)) are N-1 x 1 and the dimensions of D2 are N-1 x N-1, then these dimensions are incompatible for matrix multiplication of sign(x)*D2 (unless N = 2).

D2*sign(x), on the other hand, is valid matrix multiplication and will result in a vector with size N-1 x 1. It looks like you are then adding (diag(x)^2), which has size N-1 x N-1, so I'm not sure D2*sign(x) is what you are looking for.

Can you provide more info on what you are trying to do?