  We consider the function computation problem in a three node network with one
encoder and two decoders. The encoder has access to two correlated sources $X$
and $Y$. The encoder encodes $X^n$ and $Y^n$ into a message which is given to
two decoders. Decoder 1 and decoder 2 have access to $X$ and $Y$ respectively,
and they want to compute two functions $f(X,Y)$ and $g(X,Y)$ respectively using
the encoded message and their respective side information. We want to find the
optimum (minimum) encoding rate under the zero error and $\epsilon$-error (i.e.
vanishing error) criteria. For the special case of this problem with $f(X,Y) =
Y$ and $g(X,Y) = X$, we show that the $\epsilon$-error optimum rate is also
achievable with zero error. This result extends to a more general
`complementary delivery index coding' problem with arbitrary number of messages
and decoders. For other functions, we show that the cut-set bound is achievable
under $\epsilon$-error if $X$ and $Y$ are binary, or if the functions are from
a special class of `compatible' functions which includes the case $f=g$.
