interface
   function IDENTITY(n)
      integer n
      real(8), dimension(n,n) ::identity
   end function IDENTITY
end interface
     
interface 
   subroutine INVERSE(A,matrix_ill)
      real(8), intent(in), dimension(:,:) ::A
      logical matrix_ill
   END subroutine INVERSE
end interface
     
interface
   function MTX_CROSS_PRODUCT(A,B)
      real(8), dimension(:,:)         ::A
      real(8), dimension(:,:)         ::B
      real(8), dimension(size(A,dim=1)*size(B,dim=1),size(  &
               A,dim=2)*size(B,dim=2)) ::mtx_cross_product
   end function MTX_CROSS_PRODUCT
end interface
     
interface
   function NORM2(x)
      real(8), dimension(:) ::x
      real(8)  norm2
   end function NORM2
end interface
     
interface
   function OUTER_PRODUCT(x,y)
      real(8), dimension(:)               ::x
      real(8), dimension(:)               ::y
      real(8), dimension(size(x),size(y)) ::outer_product
   end function OUTER_PRODUCT
end interface