Though several possible derivations of the name Typhon have been suggested, the derivation remains uncertain. Some compound verbs are difficult to analyze morphologically because several derivations are plausible. There can be other derivations from nouns that are not usually considered declensions. Critics voice their strongest doubts over Bernal's approach to language and word derivations (etymologies). Directional derivatives are often used in introductory derivations of the Riemann curvature tensor. There are at least two different mathematical derivations of the Phillips curve. Most of these derivations use only elementary concepts. For examples of such derivations, see Maximum entropy probability distribution. To explain these derivations, the following notation is introduced. Word derivations occur in many languages. Indeed, none of the elementary derivations of special relativity require them. See reference "V" derivations for basic physics explanation. The inner derivations form a Lie subalgebra of formula 73. There are a large number of variants and derivations. See the article Gaussian surface for more details on how these derivations are done.