subroutine mat_w2f(mat,fname,N)
        integer, intent(in) :: N
        real(kind=8), intent(in), dimension(N,N) :: mat
        character (len=*), intent(in) :: fname
        integer :: i
        open(unit=1,file=fname)
        do i=1,N
                do j=1,N
                write(1,*), mat(i,j)
                end do
        end do
        close(1)
end subroutine mat_w2f