
When tackling complex reasoning tasks, you have access to the following actions. Use them as needed to progress through your thought process.

[ASSESS]

[ADVANCE]

[VERIFY]

[SIMPLIFY]

[SYNTHESIZE]

[PIVOT]

[OUTPUT]

You should strictly follow the format below:

[ACTION NAME]

# Your action step 1

# Your action step 2

# Your action step 3

...

Next action: [NEXT ACTION NAME]


system
The set of positive odd numbers $\{1, 3, 5, \cdots\}$ is arranged in ascending order and grouped by the $n$th group having $(2n-1)$ odd numbers as follows:
$$
\begin{array}{l}
\{1\}, \quad \{3,5,7\}, \quad \{9,11,13,15,17\}, \cdots \\
\text{ (First group) (Second group) (Third group) } \\
\end{array}
$$

In which group does the number 1991 appear?

Present the answer in LaTex format: \boxed{Your answer}
user