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carcinisation.com
| | www.logicmatters.net
8.9 parsecs away

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| | A standard menu for a first mathematical logic course might be something like this: (1) A treatment of the syntax and semantics of FOL, presenting a proof system or two, leading up to a proof of a Gödel's completeness theorem (and then a glance at e.g. the compactness theorem and some initial implications). (2) An [...]
| | xorshammer.com
15.2 parsecs away

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| | We think of a proof as being non-constructive if it proves "There exists an $latex x$ such that $latex P(x)$ without ever actually exhibiting such an $latex x$. If you want to form a system of mathematics where all proofs are constructive, one thing you can do is remove the principle of proof by contradiction:...
| | dvt.name
10.5 parsecs away

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| | Gödel's incompleteness theorems have been hailed as "the greatest mathematical discoveries of the 20th century" - indeed, the theorems apply not only to mathematics, but all formal systems and have deep implications for science, logic, computer science, philosophy, and so on. In this post, I'll give a simple but rigorous sketch of Gödel's First Incompleteness ...
| | www.jeremykun.com
87.4 parsecs away

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| Problem: Show there are finitely many primes. "Solution": Suppose to the contrary there are infinitely many primes. Let $ P$ be the set of primes, and $ S$ the set of square-free natural numbers (numbers whose prime factorization has no repeated factors). To each square-free number $ n \in S$ there corresponds a subset of primes, specifically the primes which make up $ n$'s prime factorization. Similarly, any subset $ Q \subset P$ of primes corresponds to a number in $ S$, since we can simply multiply all numbers in $ Q$ together to get a square-free number.