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Concept
Palindrome
Summary
A palindrome is a word, number, phrase, or other sequence of symbols that reads the same backwards as forwards, such as madam or racecar, the date and time 12/21/33 12:21, and the sentence: "A man, a plan, a canal – Panama". The 19-letter Finnish word saippuakivikauppias (a soapstone vendor), is the longest single-word palindrome in everyday use, while the 12-letter term tattarrattat (from James Joyce in Ulysses) is the longest in English.
The word palindrome was introduced by English poet and writer Henry Peacham in 1638. The concept of a palindrome can be dated to the 3rd-century BCE, although no examples survive; the first physical examples can be dated to the 1st-century CE with the Latin acrostic word square, the Sator Square (contains both word and sentence palindromes), and the 4th-century Greek Byzantine sentence palindrome nipson anomemata me monan opsin.
Palindromes are also found in music (the table canon and crab canon) and biological structures (most genomes include palindromic gene sequences). In automata theory, the set of all palindromes over an alphabet is a context-free language, but it is not regular.
The word palindrome was introduced by English poet and writer Henry Peacham in 1638. It is derived from the Greek roots πάλιν 'again' and δρóμος 'way, direction'; a different word is used in Greek, καρκινικός 'carcinic' (lit. crab-like) to refer to letter-by-letter reversible writing.
The ancient Greek poet Sotades (3rd-century BC) invented a form of Ionic meter called Sotadic or Sotadean verse, which is sometimes said to have been palindromic, since it is sometimes possible to make a sotadean line by reversing a dactylic hexameter.
A 1st-century Latin palindrome was found as a graffito at Pompeii. This palindrome, known as 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 also an acrostic where the first letters of each word form the first word, the second letters form the second word, and so forth.
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Pushdown automaton
In the theory of computation, a branch of theoretical computer science, a pushdown automaton (PDA) is a type of automaton that employs a stack. Pushdown automata are used in theories about what can be computed by machines. They are more capable than finite-state machines but less capable than Turing machines (see below). Deterministic pushdown automata can recognize all deterministic context-free languages while nondeterministic ones can recognize all context-free languages, with the former often used in parser design.
Automata theory
Automata theory is the study of abstract machines and automata, as well as the computational problems that can be solved using them. It is a theory in theoretical computer science. The word automata comes from the Greek word αὐτόματος, which means "self-acting, self-willed, self-moving". An automaton (automata in plural) is an abstract self-propelled computing device which follows a predetermined sequence of operations automatically. An automaton with a finite number of states is called a Finite Automaton (FA) or Finite-State Machine (FSM).
Constrained writing
Constrained writing is a literary technique in which the writer is bound by some condition that forbids certain things or imposes a pattern. Constraints are very common in poetry, which often requires the writer to use a particular verse form. Constraints on writing are common and can serve a variety of purposes. For example, a text may place restrictions on its vocabulary, e.g. Basic English, copula-free text, defining vocabulary for dictionaries, and other limited vocabularies for teaching English as a second language or to children.