Errors when trying to draw a commutative diagram using an xymatrix

Question

Why is the following code not compiling?

\[
    \xymatrix{
       A \ar[rd]^{g \circ f} \ar[r]^f & B \ar[d]^g\\
       C}
\]

I'm trying to get a commutative diagram with 3 objects (A, B and C) and 3 arrows (f, g and g ∘ f).

Answer 1

You're missing an & in the second row

\documentclass{article}
\usepackage[all,cmtip]{xy}
\begin{document}
\[
\xymatrix{
  A \ar[rd]_{g \circ f} \ar[r]^f & B \ar[d]^g\\
  & C}
\]
\end{document}

It's probably better to put the g\circ f label under the arrow.