Let Z be the set of integers. Then Z is not compact.

What sets are compact?

A set S⊆R is called compact if every sequence in S has a subsequence that converges to a point in S. One can easily show that closed intervals [a,b] are compact, and compact sets can be thought of as generalizations of such closed bounded intervals.

Is R2 compact?

As the name says itself, the smartphone is compact as it comes in a smaller display size. The phone has a 5.2-inch display size with a resolution of 1080 x 2280 pixels and IGZO type screen with 485 PPI (pixels per inch). Sharp Aquos R2 Compact comes in three different colors, black, white, and green.