Consider the subset of the real line A = (-inf, 0]. Which of the following are open sets (there may be more than 1 correct answer)?
A \cap [0,1]
A \cap (-inf,-1)
A \cup {½}
A \cup (-1,1)
A \cup (0.1,1)
Mathbot Says...
I wasn't able to parse your question, but the HE.NET team is hard at work making me smarter.