Slate (writing) Slate (writing) A slate is a thin piece of hard flat material, such as the rock also called slate, that is used as a medium for writing. The rock is “a metamorphic rock created by the recrystallization of the minerals in shale from clay to parallel-aligned, flat, flake-like minerals such as mica”. The writing slate consisted of a piece of slate, typically either 4x6 inches or 7x10 inches, encased in a wooden frame. A precise date range for writing slates of this type has not been established. Usually, a piece of cloth or slate sponge was used to clean it
Change of variables (PDE) can thus be visualized as rotation on torii. As a particular example, consider the simple harmonic oscillator, with formula_70 and formula_71, with Hamiltonian formula_72. This system can be rewritten as formula_73, formula_74, where formula_75 and formula_76 are the canonical polar coordinates: formula_77 and formula_78. See V. I. Arnold, `Mathematical Methods of Classical Mechanics', for more details. Change of variables (PDE) Often a partial differential equation can be reduced to a simpler form with a known solution by a suitable change of variables. The article discusses change of variable for PDEs below in two ways: For example, the following simplified form