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Motivation

(n k) is the number of ways

• to choose a k element set
• from an n-element set.

Example:

The set {0, 1, 2, 3} has 6 = (4 2) subsets with 2 elements:

{0, 1}, {0, 2}, {0, 3}, {1, 2}, {1, 3}, {2, 3}.