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      1 年前

      × is the cartesian product and = {x, {x,y}} is the ordered pair of x and y. (i.e., if x is in X and y is in Y, then is the corresponding element of the cartesian product X × Y). hope this helps

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          1 年前

          it’s the “order type” of a well ordering on a set. so, given a set X with a total ordering R, type(X,R) is the unique ordinal isomorphic to (X,R)

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              1 年前

              yeah but sometimes when the textbook authors are feeling particularly mischievous they’ll just put them in random places. and sometimes they’ll even skip the proofs but keep the square.