[高微] open set

[benson2826 (benson)]

let s={(x,y)屬於R^2 l xy>1 }

show that s is open

高手幫幫我

高微的習題

謝謝

--
※ 發信站: 批踢踢實業坊(ptt.cc) 
◆ From: 118.233.72.20
推 bolue       :R^2 \ s = 圓盤(closed) 所以s是open 有錯請指正
→ tasukuchiyan:R^2 \ s 非圓盤
→ linpotso    :Let f(x,y)=xy is continuous func. A=(1,infinity)
→ linpotso    :f^(-1)(A)=S is also open
→ linpotso    :but you need to prove f is continuous