/sci/ - Science & Math

>>11718546
a circle can be realized as [0,1] with endpoints glued together
a 2-sphere can be realized as the unit 2-disc with its boundary circle collapsed to a point (picture)
a 3-sphere can be realized as the unit 3-disc with its boundary collapsed to a point

you can picture 3-torus as a cube where passing through the boundary moves you to the opposite side.
3-sphere is similar, but you can walk through the boundary and appear anywhere on the boundary you like. because the whole boundary is a single point now.