program snsimp
c
c
c This example program is intended to illustrate the
c simplest case of using ARPACK in considerable detail.
c This code may be used to understand basic usage of ARPACK
c and as a template for creating an interface to ARPACK.
c
c This code shows how to use ARPACK to find a few eigenvalues
c (lambda) and corresponding eigenvectors (x) for the standard
c eigenvalue problem:
c
c A*x = lambda*x
c
c where A is a n by n real nonsymmetric matrix.
c
c The main points illustrated here are
c
c 1) How to declare sufficient memory to find NEV
c eigenvalues of largest magnitude. Other options
c are available.
c
c 2) Illustration of the reverse communication interface
c needed to utilize the top level ARPACK routine SNAUPD
c that computes the quantities needed to construct
c the desired eigenvalues and eigenvectors(if requested).
c
c 3) How to extract the desired eigenvalues and eigenvectors
c using the ARPACK routine SNEUPD.
c
c The only thing that must be supplied in order to use this
c routine on your problem is to change the array dimensions
c appropriately, to specify WHICH eigenvalues you want to compute
c and to supply a matrix-vector product
c
c w <- Av
c
c in place of the call to AV( ) below.
c
c Once usage of this routine is understood, you may wish to explore
c the other available options to improve convergence, to solve generalized
c problems, etc. Look at the file ex-nonsym.doc in DOCUMENTS directory.
c This codes implements
c
c\Example-1
c ... Suppose we want to solve A*x = lambda*x in regular mode,
c where A is obtained from the standard central difference
c discretization of the convection-diffusion operator
c (Laplacian u) + rho*(du / dx)
c on the unit square, with zero Dirichlet boundary condition.
c
c ... OP = A and B = I.
c ... Assume "call av (nx,x,y)" computes y = A*x
c ... Use mode 1 of SNAUPD.
c
c\BeginLib
c
c\Routines called:
c snaupd ARPACK reverse communication interface routine.
c sneupd ARPACK routine that returns Ritz values and (optionally)
c Ritz vectors.
c slapy2 LAPACK routine to compute sqrt(x**2+y**2) carefully.
c saxpy Level 1 BLAS that computes y <- alpha*x+y.
c snrm2 Level 1 BLAS that computes the norm of a vector.
c av Matrix vector multiplication routine that computes A*x.
c tv Matrix vector multiplication routine that computes T*x,
c where T is a tridiagonal matrix. It is used in routine
c av.
c
c\Author
c Richard Lehoucq
c Danny Sorensen
c Chao Yang
c Dept. of Computational &
c Applied Mathematics
c Rice University
c Houston, Texas
c
c\SCCS Information: @(#)
c FILE: nsimp.F SID: 2.5 DATE OF SID: 10/17/00 RELEASE: 2
c
c\Remarks
c 1. None
c
c\EndLib
c---------------------------------------------------------------------------
c
c %------------------------------------------------------%
c | Storage Declarations: |
c | |
c | The maximum dimensions for all arrays are |
c | set here to accommodate a problem size of |
c | N .le. MAXN |
c | |
c | NEV is the number of eigenvalues requested. |
c | See specifications for ARPACK usage below. |
c | |
c | NCV is the largest number of basis vectors that will |
c | be used in the Implicitly Restarted Arnoldi |
c | Process. Work per major iteration is |
c | proportional to N*NCV*NCV. |
c | |
c | You must set: |
c | |
c | MAXN: Maximum dimension of the A allowed. |
c | MAXNEV: Maximum NEV allowed. |
c | MAXNCV: Maximum NCV allowed. |
c %------------------------------------------------------%
c
integer maxn, maxnev, maxncv, ldv
parameter (maxn=256, maxnev=12, maxncv=30, ldv=maxn)
c
c %--------------%
c | Local Arrays |
c %--------------%
c
integer iparam(11), ipntr(14)
logical select(maxncv)
Real
& ax(maxn), d(maxncv,3), resid(maxn),
& v(ldv,maxncv), workd(3*maxn),
& workev(3*maxncv),
& workl(3*maxncv*maxncv+6*maxncv)
c
c %---------------%
c | Local Scalars |
c %---------------%
c
character bmat*1, which*2
integer ido, n, nx, nev, ncv, lworkl, info, ierr,
& j, ishfts, maxitr, mode1, nconv
Real
& tol, sigmar, sigmai
logical first, rvec
c
c %------------%
c | Parameters |
c %------------%
c
Real
& zero
parameter (zero = 0.0E+0)
c
c %-----------------------------%
c | BLAS & LAPACK routines used |
c %-----------------------------%
c
Real
& slapy2, snrm2
external slapy2, snrm2, saxpy
c
c %--------------------%
c | Intrinsic function |
c %--------------------%
c
intrinsic abs
c
c %-----------------------%
c | Executable Statements |
c %-----------------------%
c
c %-------------------------------------------------%
c | The following include statement and assignments |
c | initiate trace output from the internal |
c | actions of ARPACK. See debug.doc in the |
c | DOCUMENTS directory for usage. Initially, the |
c | most useful information will be a breakdown of |
c | time spent in the various stages of computation |
c | given by setting mnaupd = 1. |
c %-------------------------------------------------%
c
include 'debug.h'
ndigit = -3
logfil = 6
mnaitr = 0
mnapps = 0
mnaupd = 1
mnaup2 = 0
mneigh = 0
mneupd = 0
c
c %-------------------------------------------------%
c | The following sets dimensions for this problem. |
c %-------------------------------------------------%
c
nx = 10
n = nx*nx
c
c %-----------------------------------------------%
c | |
c | Specifications for ARPACK usage are set |
c | below: |
c | |
c | 1) NEV = 4 asks for 4 eigenvalues to be |
c | computed. |
c | |
c | 2) NCV = 20 sets the length of the Arnoldi |
c | factorization. |
c | |
c | 3) This is a standard problem. |
c | (indicated by bmat = 'I') |
c | |
c | 4) Ask for the NEV eigenvalues of |
c | largest magnitude. |
c | (indicated by which = 'LM') |
c | See documentation in SNAUPD for the |
c | other options SM, LR, SR, LI, SI. |
c | |
c | Note: NEV and NCV must satisfy the following |
c | conditions: |
c | NEV <= MAXNEV |
c | NEV + 2 <= NCV <= MAXNCV |
c | |
c %-----------------------------------------------%
c
nev = 4
ncv = 20
bmat = 'I'
which = 'LM'
c
if ( n .gt. maxn ) then
print *, ' ERROR with _NSIMP: N is greater than MAXN '
go to 9000
else if ( nev .gt. maxnev ) then
print *, ' ERROR with _NSIMP: NEV is greater than MAXNEV '
go to 9000
else if ( ncv .gt. maxncv ) then
print *, ' ERROR with _NSIMP: NCV is greater than MAXNCV '
go to 9000
end if
c
c %-----------------------------------------------------%
c | |
c | Specification of stopping rules and initial |
c | conditions before calling SNAUPD |
c | |
c | TOL determines the stopping criterion. |
c | |
c | Expect |
c | abs(lambdaC - lambdaT) < TOL*abs(lambdaC) |
c | computed true |
c | |
c | If TOL .le. 0, then TOL <- macheps |
c | (machine precision) is used. |
c | |
c | IDO is the REVERSE COMMUNICATION parameter |
c | used to specify actions to be taken on return |
c | from SNAUPD. (see usage below) |
c | |
c | It MUST initially be set to 0 before the first |
c | call to SNAUPD. |
c | |
c | INFO on entry specifies starting vector information |
c | and on return indicates error codes |
c | |
c | Initially, setting INFO=0 indicates that a |
c | random starting vector is requested to |
c | start the ARNOLDI iteration. Setting INFO to |
c | a nonzero value on the initial call is used |
c | if you want to specify your own starting |
c | vector (This vector must be placed in RESID). |
c | |
c | The work array WORKL is used in SNAUPD as |
c | workspace. Its dimension LWORKL is set as |
c | illustrated below. |
c | |
c %-----------------------------------------------------%
c
lworkl = 3*ncv**2+6*ncv
tol = zero
ido = 0
info = 0
c
c %---------------------------------------------------%
c | Specification of Algorithm Mode: |
c | |
c | This program uses the exact shift strategy |
c | (indicated by setting IPARAM(1) = 1). |
c | IPARAM(3) specifies the maximum number of Arnoldi |
c | iterations allowed. Mode 1 of SNAUPD is used |
c | (IPARAM(7) = 1). All these options can be changed |
c | by the user. For details see the documentation in |
c | SNAUPD. |
c %---------------------------------------------------%
c
ishfts = 1
maxitr = 300
mode1 = 1
c
iparam(1) = ishfts
c
iparam(3) = maxitr
c
iparam(7) = mode1
c
c %-------------------------------------------%
c | M A I N L O O P (Reverse communication) |
c %-------------------------------------------%
c
10 continue
c
c %---------------------------------------------%
c | Repeatedly call the routine SNAUPD and take |
c | actions indicated by parameter IDO until |
c | either convergence is indicated or maxitr |
c | has been exceeded. |
c %---------------------------------------------%
c
call snaupd ( ido, bmat, n, which, nev, tol, resid, ncv,
& v, ldv, iparam, ipntr, workd, workl, lworkl,
& info )
c
if (ido .eq. -1 .or. ido .eq. 1) then
c
c %-------------------------------------------%
c | Perform matrix vector multiplication |
c | y <--- Op*x |
c | The user should supply his/her own |
c | matrix vector multiplication routine here |
c | that takes workd(ipntr(1)) as the input |
c | vector, and return the matrix vector |
c | product to workd(ipntr(2)). |
c %-------------------------------------------%
c
call av (nx, workd(ipntr(1)), workd(ipntr(2)))
c
c %-----------------------------------------%
c | L O O P B A C K to call SNAUPD again. |
c %-----------------------------------------%
c
go to 10
c
endif
c
c %----------------------------------------%
c | Either we have convergence or there is |
c | an error. |
c %----------------------------------------%
c
if ( info .lt. 0 ) then
c
c %--------------------------%
c | Error message, check the |
c | documentation in SNAUPD. |
c %--------------------------%
c
print *, ' '
print *, ' Error with _naupd, info = ',info
print *, ' Check the documentation of _naupd'
print *, ' '
c
else
c
c %-------------------------------------------%
c | No fatal errors occurred. |
c | Post-Process using SNEUPD. |
c | |
c | Computed eigenvalues may be extracted. |
c | |
c | Eigenvectors may be also computed now if |
c | desired. (indicated by rvec = .true.) |
c | |
c | The routine SNEUPD now called to do this |
c | post processing (Other modes may require |
c | more complicated post processing than |
c | mode1,) |
c | |
c %-------------------------------------------%
c
rvec = .true.
c
call sneupd ( rvec, 'A', select, d, d(1,2), v, ldv,
& sigmar, sigmai, workev, bmat, n, which, nev, tol,
& resid, ncv, v, ldv, iparam, ipntr, workd, workl,
& lworkl, ierr )
c
c %------------------------------------------------%
c | The real parts of the eigenvalues are returned |
c | in the first column of the two dimensional |
c | array D, and the IMAGINARY part are returned |
c | in the second column of D. The corresponding |
c | eigenvectors are returned in the first |
c | NCONV (= IPARAM(5)) columns of the two |
c | dimensional array V if requested. Otherwise, |
c | an orthogonal basis for the invariant subspace |
c | corresponding to the eigenvalues in D is |
c | returned in V. |
c %------------------------------------------------%
c
if ( ierr .ne. 0) then
c
c %------------------------------------%
c | Error condition: |
c | Check the documentation of SNEUPD. |
c %------------------------------------%
c
print *, ' '
print *, ' Error with _neupd, info = ', ierr
print *, ' Check the documentation of _neupd. '
print *, ' '
c
else
c
first = .true.
nconv = iparam(5)
do 20 j=1, nconv
c
c %---------------------------%
c | Compute the residual norm |
c | |
c | || A*x - lambda*x || |
c | |
c | for the NCONV accurately |
c | computed eigenvalues and |
c | eigenvectors. (IPARAM(5) |
c | indicates how many are |
c | accurate to the requested |
c | tolerance) |
c %---------------------------%
c
if (d(j,2) .eq. zero) then
c
c %--------------------%
c | Ritz value is real |
c %--------------------%
c
call av(nx, v(1,j), ax)
call saxpy(n, -d(j,1), v(1,j), 1, ax, 1)
d(j,3) = snrm2(n, ax, 1)
d(j,3) = d(j,3) / abs(d(j,1))
c
else if (first) then
c
c %------------------------%
c | Ritz value is complex. |
c | Residual of one Ritz |
c | value of the conjugate |
c | pair is computed. |
c %------------------------%
c
call av(nx, v(1,j), ax)
call saxpy(n, -d(j,1), v(1,j), 1, ax, 1)
call saxpy(n, d(j,2), v(1,j+1), 1, ax, 1)
d(j,3) = snrm2(n, ax, 1)
call av(nx, v(1,j+1), ax)
call saxpy(n, -d(j,2), v(1,j), 1, ax, 1)
call saxpy(n, -d(j,1), v(1,j+1), 1, ax, 1)
d(j,3) = slapy2( d(j,3), snrm2(n, ax, 1) )
d(j,3) = d(j,3) / slapy2(d(j,1),d(j,2))
d(j+1,3) = d(j,3)
first = .false.
else
first = .true.
end if
c
20 continue
c
c %-----------------------------%
c | Display computed residuals. |
c %-----------------------------%
c
call smout(6, nconv, 3, d, maxncv, -6,
& 'Ritz values (Real, Imag) and residual residuals')
end if
c
c %-------------------------------------------%
c | Print additional convergence information. |
c %-------------------------------------------%
c
if ( info .eq. 1) then
print *, ' '
print *, ' Maximum number of iterations reached.'
print *, ' '
else if ( info .eq. 3) then
print *, ' '
print *, ' No shifts could be applied during implicit',
& ' Arnoldi update, try increasing NCV.'
print *, ' '
end if
c
print *, ' '
print *, ' _NSIMP '
print *, ' ====== '
print *, ' '
print *, ' Size of the matrix is ', n
print *, ' The number of Ritz values requested is ', nev
print *, ' The number of Arnoldi vectors generated',
& ' (NCV) is ', ncv
print *, ' What portion of the spectrum: ', which
print *, ' The number of converged Ritz values is ',
& nconv
print *, ' The number of Implicit Arnoldi update',
& ' iterations taken is ', iparam(3)
print *, ' The number of OP*x is ', iparam(9)
print *, ' The convergence criterion is ', tol
print *, ' '
c
end if
c
c %---------------------------%
c | Done with program snsimp. |
c %---------------------------%
c
9000 continue
c
end
c
c==========================================================================
c
c matrix vector subroutine
c
c The matrix used is the 2 dimensional convection-diffusion
c operator discretized using central difference.
c
subroutine av (nx, v, w)
integer nx, j, lo
Real
& v(nx*nx), w(nx*nx), one, h2
parameter (one = 1.0E+0)
external saxpy, tv
c
c Computes w <--- OP*v, where OP is the nx*nx by nx*nx block
c tridiagonal matrix
c
c | T -I |
c |-I T -I |
c OP = | -I T |
c | ... -I|
c | -I T|
c
c derived from the standard central difference discretization
c of the 2 dimensional convection-diffusion operator
c (Laplacian u) + rho*(du/dx) on a unit square with zero boundary
c condition.
c
c When rho*h/2 <= 1, the discrete convection-diffusion operator
c has real eigenvalues. When rho*h/2 > 1, it has complex
c eigenvalues.
c
c The subroutine TV is called to computed y<---T*x.
c
c
h2 = one / real((nx+1)*(nx+1))
c
call tv(nx,v(1),w(1))
call saxpy(nx, -one/h2, v(nx+1), 1, w(1), 1)
c
do 10 j = 2, nx-1
lo = (j-1)*nx
call tv(nx, v(lo+1), w(lo+1))
call saxpy(nx, -one/h2, v(lo-nx+1), 1, w(lo+1), 1)
call saxpy(nx, -one/h2, v(lo+nx+1), 1, w(lo+1), 1)
10 continue
c
lo = (nx-1)*nx
call tv(nx, v(lo+1), w(lo+1))
call saxpy(nx, -one/h2, v(lo-nx+1), 1, w(lo+1), 1)
c
return
end
c=========================================================================
subroutine tv (nx, x, y)
c
integer nx, j
Real
& x(nx), y(nx), h, dd, dl, du, h2
c
Real
& one, rho
parameter (one = 1.0E+0, rho = 1.0E+2)
c
c Compute the matrix vector multiplication y<---T*x
c where T is a nx by nx tridiagonal matrix with DD on the
c diagonal, DL on the subdiagonal, and DU on the superdiagonal.
c
c When rho*h/2 <= 1, the discrete convection-diffusion operator
c has real eigenvalues. When rho*h/2 > 1, it has complex
c eigenvalues.
c
h = one / real(nx+1)
h2 = h*h
dd = 4.0E+0 / h2
dl = -one/h2 - 5.0E-1*rho/h
du = -one/h2 + 5.0E-1*rho/h
c
y(1) = dd*x(1) + du*x(2)
do 10 j = 2,nx-1
y(j) = dl*x(j-1) + dd*x(j) + du*x(j+1)
10 continue
y(nx) = dl*x(nx-1) + dd*x(nx)
return
end
