Problems with using Eigs for a function

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Emil Ringh
Emil Ringh 2015-11-16
评论： Emil Ringh 2015-11-17
Hi,
I have problems with the interface of eigs(). I have a matrix T
T= [A B
C D]
where matrix vector multiplication can be implemented with FFT for the block D, and the rest are sparse. Hence I have created a function for T rather than a matrix. I want to compute some of the eigenvalues of T, but have problems with the interface of eigs(). I have tried different ways of calling eigs but it always gives me errors.
Note: I did read the manual and when implementing T as an ordinary matrix it works fine... I'm also aware of that the function sent to eigs should implement "T\b" but now I'm just trying to figure out the interface.
When T is implemented as a sparse matrix, the following call works fine
[v_initial, eigenvalue] = eigs(T_old, 1, 'sm');
But when I use T as a function I get the following error:
Error using eigs/checkInputs (line 496)
Generalized matrix B must be the same size as A.
Error in eigs (line 93)
[A,Amatrix,isrealprob,issymA,n,B,classAB,k,eigs_sigma,whch, ...
In the debugger I can see that the matrix B now has gotten the value 'sm'. Just because I shifted from explicit matrix to function, the interface changed...
Question: How is the interface when you use a function instead of an explicit matrix?

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采纳的回答

Steven Lord
Steven Lord 2015-11-16
Replace the A matrix input with the two inputs AFUN and N. You need to tell MATLAB not only the function it should use to compute but also how large a vector X it should pass into that function as demonstrated by the use of the extra "n" input in the second example in the EIGS documentation as compared with the first example.

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