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### [Lapack] What's the best lapack routine to use?

 ```I agree with Julien's advice. But you should know that if you have a zero or very tiny eigenvalue, roundoff may cause these routines to return a (tiny) negative eigenvalue. The easiest thing to do is simply replace these negative values by zero (as long as you are certain your input matrix is positive semidefinite). Jim Demmel On 1/15/15, 8:08 AM, Langou, Julien wrote: ``````Do you want all the eigenvalues or just a few of them? Is the matrix dense or is the matrix sparse? If you have a dense symmetric matrix and want all of its eigenvalue then you can use LAPACK subroutines: DSYEV, DSYEVD, DSYEVR, or DSYEVX. Which is the best? DSYEV is fine enough if you want only the eigenvalues (and not the eigenvectors). DSYEVR or DSYEVD are good for eigenvector computation. Which one is better kind of depends on the platform and the problem at hand. Both are good enough. Cheers, Julien. On 1/14/15, 6:38 PM, "Kevin Cahill" wrote: ``````What's the best lapack routine to use to find the eigenvalues of a huge real symmetric matrix whose eigenvalues are positive or zero? Best wishes, Kevin ``````_______________________________________________ Lapack mailing list Lapack@Domain.Removed http://lists.eecs.utk.edu/mailman/listinfo/lapack `````` ```
 Current Thread [Lapack] What's the best lapack routine to use?, Kevin Cahill [Lapack] What's the best lapack routine to use?, Langou, Julien [Lapack] What's the best lapack routine to use?, James Demmel <= Message not available [Lapack] What's the best lapack routine to use?, Langou, Julien [Lapack] What's the best lapack routine to use?, Lawrence Mulholland

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