Could you tell me if you understand and/or remember those definitions (where a and b denoting arbitrary sets):

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(a INTERSECTION b) = {x SUCH-THAT (x BELONGS-TO a) and (x BELONGS-TO b)} (a UNION b) = {x SUCH THAT (x BELONGS-TO a) or (x BELONGS-TO b)} Can you compute {1, 2, 7, 789} UNION {1, 2, 7, 5678} = ? 1,2,7,789, 5678 {1, 2, 7, 789} INTERSECTION {1, 2, 7, 5678} = ? 1, 2, 7, 789 Do you remember the empty set? Can you compute: {1, 2} UNION { } = ? 1,2 {1} UNION { } = ? { } {1, 2, 3} UNION {1, 2, 3} = ? 1,2,3 { } UNION { } = ? { } {1, 2} INTERSECTION { } = ? { } {1} INTERSECTION { } = ? { } {1, 2, 3} INTERSECTION {1, 2, 3} = ? 1, 2 3 { } INTERSECTION { } = ? { } Now, an important distinction which will follow us through ... forever. I suggest you read attentively the next two paragraphs two times before breakfast, every day for one week. :), Really take all your time. It concerns the notion of operation, and relation. INTERSECTION and UNION, are operations on sets, like addition (+, or PLUS) and multiplication (*, or TIMES) are operation on numbers. This means, typically, that, if x and y denote numbers, then x + y, and x * y, will denote, or are equal to, numbers. For example 3 + 4 is equal to 7. Similarly, if x and y denotes, or are equal, to sets, then x INTERSECTION y denotes, or is equal to, some set. For example {1,2} INTERSECTION {2, 7} is equal to some set, actually the set {2}. OK? Operations are important, as you can guess, but relations are as well important. Operations lead to new elements, new objects. From the numbers 2 and 3, you get the element 5. Relations pertains or does not pertain, or equivalently, leads to true or false. Example. The relation LESS-THAN, among the numbers. (x LESS-THAN y) is true if x is less than y. So (3 LESS-THAN 56) is true, and (56 LESS-THAN 3) is false. An important relation pertaining on sets is the relation of inclusion, or of being a subset of a set. By definition a set x will be said included in y (or be said subset of y), when all the elements of x are among the elements of y. We will write (x INCLUDED-IN y) when the set x is included in the set y. For example, the set {1, 2} is included in the set {3, 2, 1}, but is not included in the set {3, 1}. Exercise: in the following, what is true or false? 45 LESS-THAN 67 true 0 LESS-THAN 1 true 999 LESS-THAN 4 false {1, 2, 3} INCLUDED-IN {4, 1, 5, 2, 3, 8} true {1} INCLUDED-IN {1, 2} true oops, I must go. You are lucky ;) Bruno http://iridia.ulb.ac.be/~marchal/ --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "Everything List" group. To post to this group, send email to everything-list@googlegroups.com To unsubscribe from this group, send email to everything-list+unsubscr...@googlegroups.com For more options, visit this group at http://groups.google.com/group/everything-list?hl=en -~----------~----~----~----~------~----~------~--~---