:numbered: :latex: == A Short Math Article ++++ \( %% Blackboard bold \def\NN{\mathbb{N}} \def\QQ{\mathbb{Q}} \def\RR{\mathbb{R}} \def\ZZ{\mathbb{Z}} %% Arrows, sets, etc. \newcommand{\set}[1]{ \{\,#1\, \} } \newcommand{\sett}[2]{ \{\,#1\, \mid\, #2\, \} } \newcommand{\Set}[1]{ \Big\{\,#1\, \Big\} } \newcommand{\Sett}[2]{ \Big\{\,#1\, \Big\vert\; #2\, \Big\} } \newcommand{\mapright}[1]{\ \smash{ \mathop{\longrightarrow}\limits^{#1}}\ } \) ++++ This "article" is a demonstration of how one can write mathematics in Noteshare. Use arbitrary LaTeX inside the traditional dollar sign, bracket delimiters, and a form of the LaTeX environment. Equation numbering and cross-referencing is supported, as is the numbered theorem environment.footnote:[We are working on a prototype asciidoctor-to-latex converter. At the present moment it is adequate to convert this file into LaTeX. Stay tuned!] [quote] -- _Observe the footnote reference in the previous paragraph. The footnote is *way*at the bottom. However, you can click on it it reference it._ -- === Pythagorean triples One of the first real pieces of mathematics we learn is this: [env.theorem#th-pyth] -- _Let $a$, $b$, and $c$ be the sides of a right triangle, where $c$ is the hypotenuse. Then $a^2 + b^2 = c^2$._ -- The Pythagorean theorem suggests an equation, [env.equation#eq-pyth] -- x^2 + y^2 = z^2 -- If we demand that the unknowns be integers, then this is a _Diophantine equation_. We all know one solution, to equation <>, the 3-4-5 triangle. However, there are many more, in fact, infinitely many. One way of generating more solutions is to rescale existing ones. Thus 6-8-10 is a solution. However, what is interesting is that there are infinitely many _dissimilar solutions_. One is 5-12-13. Quite remarkably, there is a clay tablet (Plympton 322) from Mesopotamia, dated to about 1800 BC, that contains a list of 15 such "Pythagorean triples." See http://en.wikipedia.org/wiki/Plimpton_322[Wikipedia]. It is unlikely that the Babylonian mathematicians, despite their sophistication and skill, knew that equation <> has infinitely many solutions. This fact is equivalent to the statement that the unit circle centered at the origin has infinitely many points with rational coordinates. For more information on Pythagorean triplets, see http://en.wikipedia.org/wiki/Pythagorean_triple[Wikipedia]. === Another result from ancient times [env.theorem#th-primes] -- _There are infinitely many primes._ -- .Proof -- Suppose that there are only finitely many primes, say $p_1, p_2, \ldots, p_N$. Let -- [env.equation#eq-infprimes] -- Q = p_1p_2 \cdots p_N + 1 -- This number is is greater than the greatest prime, $p_N$. Therefore it is composite, and therefore it is divisible by $p_i$ for some $i$. But the remainder of $Q$ upon division by $p_i$ is 1, a contradiction. *Q.E.D.* === The work of Fermat Let us consider <> once again. Pierre Fermat asked whether this family of Diophantine equations $x^d + y^d = z^d$, where parameter $d$ is greater than two, have any other than the obvious solutions, e.g., $x, y, z = 1, 0 ,1$ where one variable is zero. Fermat conjectured that the answer was no, and he wrote in the margin of Diophantus' treatise that he had a marvelous proof of this fact, alas, too long to fit int he space available. More than 300 years later, Andrew Wiles, using techniques developed only in the twentieth century, gave a proof of the theorem of Fermat. === The work of Georg Cantor Georg Cantor introduced the notion of _set_, e.g., [env.equation#eq-set-integers] -- \NN = \set{ \text{integers} } = \set{ 1, 2, 3, \ldots } -- Story to be continued ... However, let's check that our cross-references work. We learned about the Pythagorean formula <>, and the number of solutions it has in <>. We also learned in <> that prime numbers are quite abundant in Nature: there are infinitely many of them. Our last, incomplete section, featuring the set of integers <>, is devoted to the work of Georg Cantor.