Two sets are equal if they contain the same elements. I. e., sets A and B are equal if ∀x[x ∈ A ↔ x ∈ B]. Notation: A = B. Recall: Sets are unordered and we do not distinguish between repeated elements. So: {1, 1, 1} = {1}, and {a, b, c} = {b, a, c}.
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