LaTex: eqnarray numbering eqs as 1a, 1b, 1c

[Saladsamurai]

Hi folks, I am making some great progress in LaTeX, however this is something I am having trouble finding. I think that I am just unsure of what keywords to search. I have the following equation array:

\begin{eqnarray}
 y(x) &=& \sum_0^{\infty}a_nx^{n+r}     \label{eqn: frobenius 6}\\
 \Rightarrow y'(x) &=& \sum_0^{\infty}(n+r)a_nx^{n+r - 1}	\label{eqn: frobenius 7}  \\
 \Rightarrow y''(x) &=& \sum_0^{\infty}(n+r-1)(n+r)a_nx^{n+r - 2}	\label{eqn: frobenius 8} 
 \label{eqn: frobenius 8}
\end{eqnarray}

which produces the following

Instead of numbering each equation as a new number, I would like it to say (4.13a), (4.13b),(4.13c).

I know this is possible; I am just unsure of the syntax.

Any thoughts? Thanks!

 

[Simon_Tyler]

What you want is subequations. You should also use the align environment instead of eqnarray, as it has better spacing -- see the links in this http://tex.stackexchange.com/questions/196/eqnarray-vs-align" .

\begin{subequations}
\begin{align}
 y(x) &= \sum_0^{\infty}a_nx^{n+r}     \label{eqn: frobenius 6}\\
 \implies y'(x) &= \sum_0^{\infty}(n+r)a_nx^{n+r - 1}	\label{eqn: frobenius 7}  \\
 \implies y''(x) &= \sum_0^{\infty}(n+r-1)(n+r)a_nx^{n+r - 2}	\label{eqn: frobenius 8} 
\end{align}
\end{subequations}

produces the attached image (it didn't render on physics forums properly, so I used http://docs.latexlab.org/ ).