- The World Factbook
The World Factbook provides basic intelligence on the history, people, government, economy, energy, geography, environment, communications, transportation, military, terrorism, and transnational issues for 265 world entities.
- Bantu-speaking people settled in the area now called Angola in 6th century A.D.; by the 10th century various Bantu groups had established kingdoms, of which Kongo became the most powerful. From the late-14th to the mid-19th century, a Kingdom of Kongo stretched across central Africa from present-day northern Angola into the current Congo republics.
- Klein bottle (Wikipedia)
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.