A nine-region division of the United States is discussed and compared with the U.S. Census and BEA (Bureau of Economic Analysis) regional groupings. The discussion includes the population density of the regions as well as the inequality of the population densities between and within regions

Vectors and matrices played a minor role in the econometric literature published before Second World War, but they have become an indispensable tool in the past several decades. Part of this development results from the importance of matrix tools for the statistical component of econometrics;...