推 hanabiz :對 08/25 18:43
→ Sfly :不對 08/25 19:44
→ Sfly :ex. f(x)=|x| and G=(0,2)U{-1}. 08/25 19:45
推 hanabiz :S大是對的 08/25 19:56
→ hanabiz :不過改成 Then the set f^(-1)f(G) is always open 08/25 19:56
→ hanabiz :就對了 08/25 19:57
→ MailMath :抱歉問一下 是不是只要加f is 1-1 就會對? 08/25 20:44
→ MailMath :先謝謝hanabiz跟Sfly !! 08/25 20:45
→ Sfly :加了1-1當然就對了 因為G是f(G)的preimage,f conti. 08/25 20:59
推 hanabiz :加了1-1的話 f^(-1)f(G)就等於G了 08/25 21:27
→ hanabiz :所以就對了 08/25 21:27
→ hanabiz :note that f^(-1)f(G) need not equal G. 08/25 21:28
推 hanabiz :need not = 不一定 08/25 21:32
→ MailMath :嗯嗯 感謝了!!! 08/25 21:33
→ EdmundLiu :conti<=> the preimage of any open set is open.. 08/26 12:45