subroutine d_get_tridiag_block(Nblock,Nsize,Amat,sub,diag,over)
integer :: Nblock
integer :: Nsize
real(8),dimension(Nblock*Nsize,Nblock*Nsize) :: Amat
real(8),dimension(Nblock-1,Nsize,Nsize) :: sub
real(8),dimension(Nblock,Nsize,Nsize) :: diag
real(8),dimension(Nblock-1,Nsize,Nsize),optional :: over
real(8),dimension(Nblock-1,Nsize,Nsize) :: over_
integer :: i,j,iblock,is,js
do iblock=1,Nblock-1
do i=1,Nsize
do j=1,Nsize
is = i + (iblock-1)*Nsize
js = j + (iblock-1)*Nsize
Sub(iblock,i,j) = Amat(Nsize+is,js)
Diag(iblock,i,j) = Amat(is,js)
Over_(iblock,i,j) = Amat(is,Nsize+js)
enddo
enddo
enddo
do i=1,Nsize
do j=1,Nsize
is = i + (Nblock-1)*Nsize
js = j + (Nblock-1)*Nsize
Diag(Nblock,i,j) = Amat(is,js)
enddo
enddo
if(present(over))over=over_
end subroutine d_get_tridiag_block