Re: 3.2.3 RC2

[Dmitri A. Sergatskov]

On Wed, Sep 2, 2009 at 1:29 PM, John W. Eaton<address@hidden> wrote:

> The test is:
>
>  n = 100;
>  k = 7;
>  a = 
> sparse([3:n,1:n,1:(n-2)],[1:(n-2),1:n,3:n],[ones(1,n-2),0.4*n*ones(1,n),ones(1,n-2)]);
>  [u,s,v] = svd(full(a));
>  s = diag(s);
>  [dum, idx] = sort(abs(s));
>  s = s(idx);
>  u = u(:,idx);
>  v = v(:,idx);
>  [u2,s2,v2,flag] = svds(a,k,0);
>  s2 = diag(s2);
>  assert(flag,!1);
>  assert(s(k:-1:1), s2, 1e-10);
>
> If you run this repeatedly, do you see some successes and some
> failures?  If it does fail, do you see that the first assert sometimes
> fails, and other times it is the second?

It is alway seems to be in the second, but I get different warnings from
time to time.  Most often it is:

warning: returning fewer singular values than requested
warning: try increasing the value of sigma
error: assert (s (k:-1:1),s2,1e-10) expected
   38.034
   38.034
   38.015
   38.015
   38.004
   38.004
but got
   38.060
   38.034
   38.034
   38.015
   38.015
   38.004
   38.004
Dimensions don't match


but sometimes it is just:

error: assert (s (k:-1:1),s2,1e-10) expected
   38.060
   38.060
   38.034
   38.034
   38.015
   38.015
   38.004
but got
   38.060
   38.034
   38.034
   38.015
   38.015
   38.004
   38.004
maximum absolute error 0.0263523 exceeds tolerance 1e-10


>
> I have not tried to debug the problem yet.
>
> David, I'm copying you because you wrote svds and might be able to see
> the problem and perhaps a fix more quickly than I could.
>
> Thanks,
>
> jwe
>

Dmitri.
--