Palindrome (; "again") and ' (; "way, direction"). Palindromes date back at least to 79 AD, as a palindrome was found as a graffito at Herculaneum, a city buried by ash in that year. This palindrome, called the Sator Square, consists of a sentence written in Latin: "Sator Arepo Tenet Opera Rotas" ("The sower Arepo holds with effort the wheels"). It is remarkable for the fact that the first letters of each word form the first word, the second letters form the second word, and so forth. Hence, it can be arranged into a word square that reads in four different
Principle of least action starting point is the "action", denoted formula_1 (calligraphic S), of a physical system. It is defined as the integral of the Lagrangian "L" between two instants of time "t" and "t" - technically a functional of the "N" generalized coordinates q = ("q", "q", ... , "q") which define the configuration of the system: where the dot denotes the time derivative, and "t" is time. Mathematically the principle is where "δ" (lowercase Greek delta) means a "small" change. In words this reads: In applications the statement and definition of action are taken together: The action and Lagrangian both contain the