A number \( M \) is a least upper bound for a sequence \(\{a_n\}\) if:
- \( M \) is an upper bound for \(\{a_n\}\), meaning \( a_n \leq M \) for all \( n \).
- For any \(\epsilon > 0\), there exists some \( a_n \) such that \( M - \epsilon < a_n \).
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