Motivated by the construction of new examples of Artin-Schelter regular algebras of global dimension four, J.J.Zhang and J.Zhang (2008) introduced an algebra extension AP [y1, y2;σ, δ, τ ] of A, which they called a double Ore extension. This construction seems to be similar to that of a two-step iterated Ore extension over A. The aim of this paper is to describe those double Ore extensions whic...

According to the publication by US Minerals Survey in 1999, the total reserve of manganese ore in the world is 0.66 billion tons. See Table 1. Table 1 World Manganese ore reserve(Mn metal weight, million tons) __________________________________________ Name of counties Reserve in 1999 __________________________________________ South Africa 370 Ukraine 135 China 40 India 34 Australia 26 Gabon 20...