Duocylinder
The duocylinder, also called the double cylinder or the bidisc, is a geometric object embedded in 4-dimensional Euclidean space, defined as the Cartesian product of two disks of respective radii r1 and r2: D = { ( x , y , z , w ) | x 2 + y 2 ≤ r 1 2 , z 2 + w 2 ≤ r 2 2 } {\displaystyle D=\left\{(x,y,z,w)\,\left|\,x^{2}+y^{2}\leq r_{1}^{2},\ z^{2}+w^{2}\leq r_{2}^{2}\right.\right\}} It is similar to a cylinder in 3-space, which is the Cartesian product of a disk with a line segment. But unlike the cylinder, both hypersurfaces (of a regular duocylinder) are congruent.