subroutine kvalue(i,j,nx,ny,dkx,dky,kx,ky)
c  Subroutine KVALUE finds the wavenumber coordinates of one 
c  element of a rectangular grid from subroutine FOURN.  
c
c  Input parameters:
c    i  - index in the ky direction.
c    j  - index in the kx direction.
c    nx - dimension of grid in ky direction (a power of two).
c    ny - dimension of grid in kx direction (a power of two).
c    dkx - sample interval in the kx direction.
c    dky - sample interval in the ky direction.
c
c  Output parameters:
c    kx - the wavenumber coordinate in the kx direction.
c    ky - the wavenumber coordinate in the ky direction.
c
      real kx,ky
      nyqx=nx/2+1
      nyqy=ny/2+1
      if(j.le.nyqx)then
         kx=(j-1)*dkx
         else
            kx=(j-nx-1)*dkx
            end if
      if(i.le.nyqy)then
         ky=(i-1)*dky
         else
            ky=(i-ny-1)*dky
            end if
      return
      end