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Answers for “两个集合 A 和 B 的笛卡尔积是什么?”

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The Cartesian product of two sets A and B, denoted A×BA \times B, is the set of all possible ordered pairs (x, y) where the first element x belongs to set A and the second element y belongs to set B.

Conditions: A and B are arbitrary sets.

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No, a relation does not have to contain every possible ordered pair. By definition, a relation is a subset of the Cartesian product A×BA \times B, meaning it can include only some of the ordered pairs.

Conditions: A and B are sets.; R is a relation between A and B.

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In the definition of a relation, capital letters A and B denote the sets themselves, while lowercase letters a and b denote individual elements belonging to those sets. The ordered pair (a, b) represents a specific element of the Cartesian product A×BA \times B, where a is an element of A and b is an element of B.

Conditions: The context is the formal definition of a relation R between sets A and B.; The notation (a, b) ∈ A×BA \times B is used.

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Yes, elements of the sets can be left out of a relation. A relation is a subset of the Cartesian product, so it does not need to include every possible ordered pair.

Conditions: A relation R is defined between sets A and B.; R is a subset of A×BA \times B.

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A relation R between two sets A and B is formally defined as a subset of the Cartesian product A×BA \times B. This means that R is a collection of ordered pairs, where the first element of each pair belongs to set A and the second element belongs to set B.

Conditions: A and B are sets.; R is a binary relation between A and B.

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The Cartesian product distributes over both union and intersection from both the left and the right. Specifically, (A ∪ B) × C=(A×C)C = (A \times C) ∪ (B×CB \times C), (A ∩ B) × C=(A×C)C = (A \times C) ∩ (B×CB \times C), A × (B ∪ C) = (A×BA \times B) ∪ (A×CA \times C), and A × (B ∩ C) = (A×BA \times B) ∩ (A×CA \times C).

Conditions: A, B, and C are arbitrary sets.

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The intersection of two Cartesian products (A×BA \times B) and (C×DC \times D) is equal to the Cartesian product of the intersections of their respective component sets, i.e., (A ∩ C) × (B ∩ D).

Conditions: A, B, C, and D are arbitrary sets.

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A group action of a group GG on a set AA is a map from G×AG \times A to AA, written as g⋅ag \cdot a, that satisfies two properties: compatibility with the group operation (g1⋅(g2⋅a)=(g1g2)⋅ag_1 \cdot (g_2 \cdot a) = (g_1 g_2) \cdot a) and the identity property (1⋅a=a1 \cdot a = a).

Conditions: GG is a group.; AA is a set.; The action is a map from G×AG \times A to AA.; Property (a) holds for all g1,g2∈Gg_1, g_2 \in G and a∈Aa \in A.; Property (b) holds for all a∈Aa \in A.