>>10925704consider a 1d number line.
take
at x=1, we're in the interval, but if we move to the right at all, we'll be outside the interval. it has a boundary on the right, precisely at x=1.
now take
obviously, x=1 isn't in the interval. but where exactly is the boundary? in other words, what's the largest number in the interval?
there isn't one. we can always move an infinitesimally closer to x=1 from the left and still be in the interval.
and if something doesn't have a boundary, what is it?
