Help with diagram

Question

I do not have very much experience with the xy-pic package and I always try to figure out things by copying and pasting codes.

I am stuck in here: I would like to get an arrow from A to C with an upper name.

\xymatrix{ A \ar[r] ^a="a"  & B \ar[r] ^b="b" & C }

What I succeeded to plot is:

\xymatrix{A \ar[r] ^a="a" & B \ar[r] ^b="b" & C \ar @/^/ "a";"b" }

but I'd like the arrow to go from A to C :(

I appreciate any kind of help! Thanks a lot!!!

Answer 1

Just to offer an alternative, the tikz-cd package can also be used to produce commutative diagrams using the versatility of PGF/TikZ; here's the code producing the desired diagram:

\documentclass{article}
\usepackage{tikz-cd}

\begin{document}

\begin{tikzcd}
 A \ar{r}{a}\ar[bend left=40]{rr}{b\circ a} & B \ar{r}{b} & C
\end{tikzcd}

\end{document}

Answer 2

You can add modifiers to the arrows:

\documentclass{article}
\usepackage[all,cmtip]{xy}
\begin{document}
\xymatrix{
 A \ar[r]^a \ar@<3pt>@/^1pc/[rr]^{b\circ a} & B \ar[r]^b & C
}
\end{document}

With @/^1pc/ you state that the arrow is curved, with @<3pt> you shift it a bit up, so the arrow tips near to C don't overlap.