Symmetric matrixIn linear algebra, a symmetric matrix is a square matrix that is equal to its transpose. Formally, Because equal matrices have equal dimensions, only square matrices can be symmetric. The entries of a symmetric matrix are symmetric with respect to the main diagonal. So if denotes the entry in the th row and th column then for all indices and Every square diagonal matrix is symmetric, since all off-diagonal elements are zero. Similarly in characteristic different from 2, each diagonal element of a skew-symmetric matrix must be zero, since each is its own negative.
Phosphorus cycleThe phosphorus cycle is the biogeochemical cycle that describes the movement of phosphorus through the lithosphere, hydrosphere, and biosphere. Unlike many other biogeochemical cycles, the atmosphere does not play a significant role in the movement of phosphorus, because phosphorus and phosphorus-based compounds are usually solids at the typical ranges of temperature and pressure found on Earth. The production of phosphine gas occurs in only specialized, local conditions.
History of the Mediterranean regionThe history of the Mediterranean region and of the cultures and people of the Mediterranean Basin is important for understanding the origin and development of the Mesopotamian, Egyptian, Canaanite, Phoenician, Hebrew, Carthaginian, Minoan, Greek, Persian, Illyrian, Thracian, Etruscan, Iberian, Roman, Byzantine, Bulgarian, Arab, Berber, Ottoman, Christian and Islamic cultures. The Mediterranean Sea was the central superhighway of transport, trade and cultural exchange between diverse peoples encompassing three continents: Western Asia, North Africa, and Southern Europe.
Mediterranean tropical-like cycloneMediterranean tropical-like cyclones, often referred to as medicanes (a portmanteau of Mediterranean hurricanes) but sometimes also as Mediterranean cyclones or as Mediterranean hurricanes, are meteorological phenomena occasionally observed over the Mediterranean Sea. On a few rare occasions, some storms have been observed reaching the strength of a , on the Saffir–Simpson scale, and one storm has been recorded reaching intensity. The main societal hazard posed by medicanes is not usually from destructive winds, but through life-threatening torrential rains and flash floods.
