[Octave-bug-tracker] [bug #60924] DNAUPD did not find any eigenvalues to sufficient accuracy

[anonymous]

URL:
  <https://savannah.gnu.org/bugs/?60924>

                 Summary: DNAUPD did not find any eigenvalues to sufficient
accuracy
                 Project: GNU Octave
            Submitted by: None
            Submitted on: Fri 16 Jul 2021 05:39:47 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Unexpected Error or Warning
                  Status: None
             Assigned to: None
         Originator Name: Quentin
        Originator Email: qrobinson.math@gmail.com
             Open/Closed: Open
                 Release: 6.2.0
         Discussion Lock: Any
        Operating System: Mac OS

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Details:

The error message: 

error: eigs: error in dneupd: DNAUPD did not find any eigenvalues to
sufficient accuracy.
This should not happen.  Please, see
https://www.gnu.org/software/octave/bugs.html, and file a bug report
error: called from
    eigs at line 298 column 20


Using the eigs function in a function I wrote that does not accept input or
return output, the error message above was returned. Code from the function:

nplots = 50;
xmax = 200;
alpha = 4;
eps = 6;
N = 2^10;
h = xmax/N;
x = h*(1:N)-xmax/2;
t = 0;
u = sech(sqrt(3*alpha/(4*eps))*x).^2;
tdata = zeros(nplots+1,1);
tdata(1) = t;
num_eigs = 1;
eigenvalues = zeros(length(tdata),num_eigs);
N_eig = 2^10;
k_eig = [0:N_eig/2 -N_eig/2+1:-1]*(2*pi/(xmax/2));
ksq_eig = k_eig.^2';
u_eig = u';
A = @(f) real(-ifft( -(eps/6)^(2/3)*ksq_eig .* fft(f)) -
alpha/(4*(eps/6)^(1/3))*u_eig .* f);
[~,lam] = eigs(A,N_eig,num_eigs,'sr',struct('isreal',true));


Entering the 'clear' command in the command window before running the function
seems to 'fix' the issue...




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