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Typographical Number Theory (also known as TNT) is a formal axiomatic system describing the natural numbers that appears in Douglas Hofstadter's book Gödel, Escher, Bach. It is an implementation of Peano arithmetic that Hofstadter uses to help explain Gödel's incompleteness theorems. Like any system implementing the Peano axioms, Typographical Number Theory is capable of referring to itself (it is self-referential).
Numerals In Typographical Number Theory we do not have a distinct symbol for each natural number. Instead we use a simple, uniform way of giving a compound symbol to each natural number: The symbol S can be interpreted as "the successor of", or "the number after". Since this is, however, a number theory, such interpretations are useful, but not strict. We cannot say that because four is the successor of three that four is SSSS0, but rather that since three is the successor of two, which is the successor of one, which is the successor of zero, which we have described as 0, four can be "proved" to be SSSS0. Typographical Number Theory is designed such that everything must be proved before it can be said to be true. This is its true power, and to undermine it would be to undermine its very usefulness. Variables We need a way of referring to unspecified terms, or variables. In TNT, there exist five variables. These are a, b, c, d, e. More variables can be constructed by adding the prime symbol after them, so a are all variables. In the stricter version of TNT, known as "austere" version of TNT, only a exist. Addition and multiplication of numerals In Typographical Number Theory, the usual symbols of "+" for additions, and "·" for multiplications are used. Thus to write "b plus c", we write (b+c) and "a times d" is written as (a·d) . The parentheses are required. Any laxness would violate TNT's formation system. Also only two terms can be operated on at once. Therefore to write "a plus b plus c", we must write either ((a+b)+c) or (a+(b+c)) . Equivalency The "Equals" operator is used to denote equivalence. It is defined by the symbol "=", and takes roughly the same meaning as it usually does in mathematics. For instance, (SSS0+SSS0)=SSSSSS0 is a true statement in TNT, with the interpretation "3 plus 3 equals 6". Negation In Typographical Number Theory, negation, i.e. the turning of a statement to its opposite, is denoted by the "~" or negation operator. For instance, ~(SSS0+SSS0)=SSSSSSS0 is a true statement in TNT. By negation, this means negation in boolean logic, (logical negation) rather than simply being the opposite. For example, if I were to say "I am eating a grapefruit", the opposite is "I am not eating a grapefruit", rather than "I am eating something other than a grapefruit". Similarly "The Television is on" is negated to "The Television is not on", rather than "The Television is off". This is a subtle difference, but an important one. Quantifiers There are two quantifiers used: ∀ and ∃. is used to separate a quantifier from other quantifiers or from the rest of the formula. For example: ∀a:∀b:a+b=b+a ("For every number a and every number b, a plus b equals b plus a", or more figuratively, "Addition is commutative.") ~∃c:Sc=0 ("There does not exist a number c such that c plus one equals zero", or more figuratively, "Zero is not the successor of any (natural) number.") Atoms and propositional statements All the symbols of propositional calculus are used in Typographical Number Theory, and they retain their interpretations. Atoms are here defined as strings which amount to statements of equality, such as ~S0=SS0 . 2 plus 3 equals four: (SS0+SSS0)=SSSS0 2 plus 2 is not equal to 3: ~(SS0+SS0)=SSS0 | ||||||||
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