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
Palindrome [[set (mathematics)|set]] of all palindromes in a given [[alphabet]] is a typical example of a [[formal language|language]] that is [[context-free language|context-free]], but not [[Regular language|regular]]. This means that it is impossible for a [[finite automaton|computer with a finite amount of memory]] to reliably test for palindromes. (For practical purposes with modern computers, this limitation would apply only to impractically long letter-sequences.) In addition, the set of palindromes may not be reliably tested by a [[deterministic pushdown automaton]] which also means that they are not [[LR parser|LR(k)-parsable]] or [[LL parser|LL(k)-parsable]]. When reading a palindrome from left-to-right, it is, in essence, impossible