- In mathematics, the Klein bottle (/ˈklaɪn/) is an example of a non-orientable surface; that is, informally, a one-sided surface which, if traveled upon, could be followed back to the point of origin while flipping the traveler upside down. More formally, the Klein bottle is a two-dimensional manifold on which one cannot define a normal vector at each point that varies continuously over the whole manifold. Other related non-orientable surfaces include the Möbius strip and the real projective plane. While a Möbius strip is a surface with a boundary, a Klein bottle has no boundary. For comparison, a sphere is an orientable surface with no boundary.
- Palm Sunday (Wikipedia)
Palm Sunday is the Christian moveable feast that falls on the Sunday before Easter. The feast commemorates Christ’s triumphal entry into Jerusalem, an event mentioned in each of the four canonical Gospels. Its name originates from the palm branches waved by the crowd to greet and honor Jesus Christ as he entered the city. Palm Sunday marks the first day of Holy Week; in Western Christianity, this is the beginning of the last week of the solemn season of Lent, preceding Eastertide, while in Eastern Christianity, Holy Week commences after the conclusion of Great Lent.