[Math] Is $[0,1]$ a union of family of disjoint closed intervals

analysisgeneral-topologyreal-analysis

According to this question, $[0,1]$ cannot be written as union of countable disjoint closed sets, is the same true about (uncountable) family of disjoint closed intervals ?

Best Answer

If you allow degenerate closed intervals, $[0,1]$ can be written as the union of $2^\omega=\mathfrak c$ pairwise disjoint closed intervals:

$$[0,1]=\bigcup_{x\in[0,1]}[x,x]\;.$$

Since each non-degenerate closed interval contains a non-empty open interval, any family of pairwise disjoint closed intervals in $[0,1]$ can include at most countably many non-degenerate intervals. They cannot cover $[0,1]$, so you’ll need degenerate closed intervals to complete the cover.