CODEWITHSUNDEEP
fortranlinear-algebralapack
python_learner
python_learner

Reputation: 27

(Fortran) How do I use DSYEV from lapack to calculate eigenvalues of a (square) matrix?

I want to find the smallest eigenvalue from DSYEV and I'm not sure what I'm meant to put into the code for DSYEV.

Say my matrix A is 45x45 and I want to find its eigenvalues. So far I have:

subroutine eigenvalues()
implicit none
real(kind=8),allocatable,dimension(:,:)::A
real(kind=8),allocatable,dimension(:)::WORK, W
integer, allocatable, dimension(:)::t
integer::info,k,Z
t = shape(A)
k = t(1)
allocate(W(k))
print *, shape(M)
Z = 3*k-1
call dsyev('N','U',k,M,k,W,WORK,Z,info)
end subroutine eigenvalues

I'm not really sure what is meant by choosing to store the upper triangular matrix either. I don't know what LWORK means from the documentation still.

Upvotes: 0

Views: 2232

Answers (1)

Fl.pf.
Fl.pf.

Reputation: 345

  1. You want to calculate the eigenvalues of A but call your dsyev with M
  2. You declare, but don't allocate A
  3. You neither declare, define or allocate M

Where do your matrices come from? You may have to pass them to your subroutine if you calculate them somewhere else. Then you need to pass the dimensions also.

subroutine eigenvalues(A,k,k,eigvalues)

!calling list
integer, intent(in)             :: k
double precision, intent(inout) :: A(k,k), eigvalues(k)

!local 
double precision,allocatable :: work(:)
integer                      :: lwork,info

lwork = max(1,3*k-1)
allocate(work(lwork))


call dsyev('N','U',k,A,k,eigvalues,WORK,LWORK,info)
if(info .neq. 0) exit

Something like this (not complete example).

You will need to allocate your eigenvalue vector in the calling routine aswell, im sure you need the eigenvalues there...

WORK and LWORK don't really need to concern you. About Upper and Lower, look at your matrix A before and after your dsyev call...

Upvotes: 1

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