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73

Linear algebra is vital in multiple areas of science in general. Because linear equations are so easy to solve, practically every area of modern science contains models where equations are approximated by linear equations (using Taylor expansion arguments) and solving for the system helps the theory develop. Beginning to make a list wouldn't even be relevant ...


34

Having studied Engineering, I can tell you that Linear Algebra is fundamental and an extremely powerful tool in every single discipline of Engineering. If you are reading this and considering learning linear algebra then I will first issue you with a warning: Linear algebra is mighty stuff. You should be both manically excited and scared by the awesome ...


27

As far as I know, historically smooth manifolds were the first manifolds studied by people like Poincare, Riemann, up to Whitney. There were a few major events that caused people to take things like topological and PL manifolds seriously, but originally people were not motivated to study these kinds of objects. Here are some of the big events/ideas that ...


24

The "product" of vector spaces is more properly thought of as a direct sum, since the dimension increases linearly according to addition, $\dim(V\oplus W)=\dim(V)+\dim(W)$. Tensor products are much bigger in size than sums, since we have $\dim(V\otimes W)=\dim(V)\times\dim(W)$. In fact, in analogy to elementary arithmetic, we have distributivity $(A\oplus ...


24

I will try to give a rather elementary motivation which I believe to be close to how this identity was actually discovered. Let us consider the sine function $f(z)=\sin z$. As a function of complex argument, it has one period: $f(z+2\pi)=f(z)$. Further, it is holomorphic in the whole complex plane with only simple zeroes given by $\pi\mathbb{Z}$. It ...


22

While you are deprived of Radin here is a short paper that ties together tilings, quasicrystals and diffraction, and Radin's review of another book on these subjects. Below I interpreted your requirements loosely listing books with a unifying theme that develop ideas organically and combine approaches from different areas of mathematics and applications. ...


20

It is true, when you only work on measure theory, or algebraic number theory, or classical analysis, you are unlikely to run into anything larger than $\frak c$. But if you start working in arbitrary fields, and arbitrary modules, or arbitrary rings. Not just finitely generated, or countably generated. Then you need to have a better understanding of how ...


20

The full appreciation of Jacobi's triple product identity can not be done without some understanding of the elliptic functions. However, it is possible to develop some parts of the theory of elliptic functions without any complex analysis. Anyways back to the triple product identity, it says that $$ \sum_{n = -\infty}^{\infty}z^{n}q^{n^{2}} = \prod_{n = ...


19

Motivation can be broken up into essentially two parts: (i) Stable homotopy groups of things appear in nature, and (ii) the stable category is technically convenient. I'll try to give some indication of these below, but none of the bullet points do the topic justice... There's lots of love to be had here. Stable Homotopy Groups Appear in Nature Cobordism. ...


18

I think there is more to be found out about this subject. There are many different ways to define fractional derivatives and integrals. I do not know if these come from any deep, fundamental facts, but certainly as a generalization of various formulas. Another way to think about the subject is a list of applications or tricks involving a certain integral ...


18

I can't speak much to what traditional Spectral Graph Theory is about, but my personal research has included the study of what I call "Spectral Realizations" of graphs. A spectral realization is a special geometric realization (vertices are not-necessarily-distinct points, edges are not-necessarily-non-degenerate line segments, in some $\mathbb{R}^n$) ...


18

I don't think there is just one motivation. I will mention three. First, the Jacobi identity says precisely that the bracket is a derivation with respect to itself, where a derivation of an algebra is a map $d$ with $d(a\cdot b)=d(a)\cdot b+a \cdot d(b)$. Thus, writing $\mathrm{ad}(a)$ for the map $b \mapsto [a,b]$, the Jacobi identity may be rewritten ...


18

TL;DR version Linear algebra is your ticket to multidimensional space. Study it if you are into economics, computer graphics, physics, chemistry, statistics or anything quantitative (in today's world, that's everything). Meaning of "Linear" and why it is "Easy" Since you are asking the question, perhaps you would benefit from a discussion of what ...


17

This question already has a number of nice answers; I want to emphasize the breadth of this topic. Graphs can be represented by matrices - adjacency matrices and various flavours of Laplacian matrices. This almost immediately raises the question as to what are the connections between the spectra of these matrices and the properties of the graphs. Let's call ...


17

Topology can mean different things in mathematics, depending on the context. If a mathematician describes themselves as a topologist, this likely means that they study various kinds of shapes (technically, manifolds, or related kinds of spaces), with an eye perhaps to classifying them (a typical example being the classification of closed orientable ...


17

Taking inverses reverses the order of multiplication, so if every element is its own inverse multiplication must be commutative.


17

This is a good question. My own take on motivating quadratic reciprocity is recorded here (these are lecture notes from an undergraduate course on introductory number theory). If you look there, you will find that most of what I have said is an elaboration of the two points you bring up. I think a crisp way of explaining what QR does for you is in the ...


16

To a topologist, topological groups are interesting in their own right. The group structure actually gives us interesting topological structure, too! One interesting fact is that the fundamental group (an important topological invariant) of a topological group is Abelian, a fact that spectacularly fails to be true in general - any group can be the ...


16

The definition of modularity looks more natural to me if I think of it as follows (rather than as a modified associativity or a weakened distributivity). Given any element $a$ of a lattice $L$, there is a rather obvious way to map any element $x\in L$ to a "nearest" element $\geq a$, namely send $x$ to $a\lor x$. Think of this map as "projecting " elements ...


15

One very nice elementary application is Gosper's batting average problem: if a baseball player's (3-digit rounded) batting average is $.334$, what's the smallest number of at-bats that player could have? (Batting average is computed as (number of hits)/(at-bats).) The solution proceeds by noting that a rounded average of $.334$ corresponds to an actual ...


15

The reason that $<$concept$>$ is important is that there are enough examples of $<$concept$>$ to justify making the definition. So I will answer this by giving some examples. The first example that we come across is the real numbers, $(\mathbb{R}, +)$ (by extension all the $\mathbb{R}^n$). We see that the map $sub:\mathbb{R}^2\to \mathbb{R}$ ...


15

Quadratic reciprocity allows you to make precise certain intuitions about the primes. More precisely, it tells you that for every finite set $p_1, p_2, ... p_n$ of primes and every function $f : \{ 1, 2, ... n \} \to \{ -1, 1 \}$ there exists an arithmetic progression such that any prime $q$ in that progression satisfies $\left( \frac{p_i}{q} \right) = ...


15

You asked about a natural problem that leads to this integral. Here's a summary of the argument I give in my undergraduate probability theory class. (It's due to Dan Teague; he has the article here.) Imagine throwing a dart at the origin in the plane. You're aiming at the origin, but there is some variability in your throws. The following assumptions ...


13

I'd like to mention Spirographs. The formulas are actually rather simple, but I'm afraid that my Latex-foo is not sufficient to reproduce them here adequately. So I'll just refer to the Wikipedia page, and some example images (also from Wikipedia):


13

Nice question. I'm not sure that this will be exactly what you want, but let's go for it: consider $X = \{A,B,C,D\}$, and define $d: X \times X \to \Bbb R$ putting $d(p,p) = 0$ for all $p \in X$, $d(p,q) = d(q,p)$ for all $p,q \in X$, and: $$d(A,B) = d(A,C) = d(B,C) = 2, \qquad d(A,D) = d(B,D) = d(C,D) = 1.$$ Here, $1$ and $2$ are for simplicity (note: $1 ...


12

There is indeed some nice intuition behind these definitions, and the good news is that not even all that deep. Remember two things: First, that this cohomology all comes by the "fixed by" functor $M\to M^G$, and second, that these crossed homomorphisms come from the definition of cochains, and more directly, the coboundary operator from $n$-chains to ...


11

Fractals are always a good source of pictures. It's not too hard to explain the concept behind a fractal, and then students can enjoy the pretty depictions. Some of them are also easy for students to play with themselves --- for the Koch snowflake, the dragon curve, or the Sierpinski gasket, you don't have to know any complex function theory. Fractals can ...


11

The group axioms are intended to abstract the properties of discrete symmetries (that is, bijections from a set to itself). That is, we may define a "concrete group" to be a group of permutations of some set with composition as the group operation. An abstract group is supposed to be a version of a concrete group that does not rely on a choice of group ...


11

Spectral graph theory is a discrete analogue of spectral geometry, with the Laplacian on a graph being a discrete analogue of the Laplace-Beltrami operator on a Riemannian manifold. The Laplacian $\Delta$ can be used to write down three important differential equations, both on a graph and a Riemannian manifold: The heat equation $\frac{\partial ...


11

Here is one that comes up pretty frequently. Consider a graph $(V,E)$ where $V$ is the set of vertices and $E$ the set of edges. For two vertices $v,w\in V$ define $$ d(v,w) = \text{length of shortest path between}~v~\text{and}~w. $$ You can check this is a metric fairly easily. It has applications in discrete mathematics (where you may want to use metric ...



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