11. Observe the sequence of arrays $(1),(3,5),(7,9,11)$,
$(13,15,17,19), \cdots$. Then 2009 is in the $\qquad$ group.
$$
\begin{array}{l}
\text { Notice that 2009 is the 1005th positive odd number. } \\
1+2+\cdots+44<1005 \\
<1+2+\cdots+45,
\end{array}
$$

Then 2009 is in the 45th group.