× 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

[–] rasensprenger@feddit.de 12 points 1 year ago (1 children)

What does type() mean here?

[–] affiliate@lemmy.world 25 points 1 year ago (2 children)

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)

[–] bort@feddit.de 11 points 1 year ago (1 children)

what's with the square at the end? isn't that usually for proofs?

[–] affiliate@lemmy.world 15 points 1 year ago

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.

[–] Lemjukes@lemm.ee 8 points 1 year ago

Give it up for op actually out here answering questions like a real live teacher.

[–] Sasha@lemmy.blahaj.zone 2 points 1 year ago

Oh wow, I should know that... Thanks

[–] JackRiddle@sh.itjust.works 5 points 1 year ago

× is the cartesian product I think, no clue what the other thing is tho