Solution: The problem requires counting the number of ways to partition 4 distinct items into 2 non-empty identical subsets. This is given by the Stirling numbers of the second kind, $ S(4, 2) $. The formula for $ S(n, k) $ is $ S(n, k) = S(n-1, k-1) + k \cdot S(n-1, k) $. Using known values, $ S(4, 2) = 7 $. Thus, the number of ways is $ \boxed{7} $.

Wed, 29 Apr 2026 23:20:49 +0000

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