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 permanence Principle of permanence In mathematics, the principle of permanence is that a complex function (or functional equation) which is 0 on a set with a non-isolated point is 0 everywhere (or at least on the connected component of its domain which contains the point). There are various statements of the principle, depending on the type of function or equation considered. For one variable, the principle of permanence states that if "f"("z") is an analytic function defined on an open connected subset "U" of the complex numbers C, and there exists a convergent sequence {"a"} having a limit "L" which is