[數撚圍爐區] 全港最多數撚的POST上線啦 (31)

anal真係好正
難怪咁多人笑住咁爛grade

Let S = {0, 1, 1/2, ...}, (0, 1]\S同[0, 1]\S有咩bijection?
淨低既只要諗到S同S\{0}有咩bijection就problem solved

In general, 諗下點解任何兩個countably infinite sets都有bijection

俾d信心check一check definition
(btw你會唔會係唔記得左bijection點定義？)
identity Id: A \to A係咪both injective and surjective?
injectivity: Let a, b in A. a =/= b im唔imply Id(a) =\= Id(b)?
surjectivity: 係咪for all y in A, there exists x in A such that Id(x) = y?