  The Yangian of the Lie algebra sl_n is known to have different presentations,
in particular the RTT realisation and the Drinfel'd realisation. Using the
isomorphism between them, the explicit expressions of the comultiplication, the
antipode and the counit in the Drinfel'd realisation of the Yangian Y(sl_n) are
given. As examples, the cases of Y(sl_2) and Y(sl_3) are worked out.
