{"id":2600,"date":"2017-02-28T15:08:38","date_gmt":"2017-02-28T21:08:38","guid":{"rendered":"http:\/\/blogs.ams.org\/blogonmathblogs\/?p=2600"},"modified":"2017-03-01T10:58:18","modified_gmt":"2017-03-01T16:58:18","slug":"a-circular-approach-to-linear-algebra","status":"publish","type":"post","link":"https:\/\/blogs.ams.org\/blogonmathblogs\/2017\/02\/28\/a-circular-approach-to-linear-algebra\/","title":{"rendered":"A Circular Approach To Linear Algebra"},"content":{"rendered":"
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Sadly, this is not actually the way linear transformations work, from xkcd<\/a>.<\/p><\/div>\n

This semester I’m teaching Linear Algebra for the first time, so naturally, I am constantly on the prowl for all of the linear algebra resources the internet has to offer. To begin with, I’m using a free online textbook called Linear Algebra Done Wrong<\/a><\/em> by Sergei Treil. I’ve found that it’s a bit…intense. As a person who understands linear algebra the book is very nicely written and has a logical presentation and abundant clever examples. But for a person who has never seen linear algebra, well, let’s just say it’s a bit like diving into the deep end with no floaties on while someone shoots you with a paintball gun. <\/p>\n

Consequently, this semester has left me foraging the world wide web for supplementary resources to help my poor flailing floatieless students as they try to navigate the waters of vectors and matrices. <\/p>\n

A great place for students to begin if they are totally lost is a series of wonderful YouTube videos called the Essence of Linear Algebra<\/a><\/em>, from 3Blue1Brown. The animations really help to bring out some of the geometric intuition behind vector spaces, which can seem abstract (and sometimes totally impenetrable!) to students seeing them for the first time. Of course Khan Academy also hosts a linear algebra series<\/a>, but my students haven’t found them as helpful. <\/p>\n

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Don’t like the dog? You can also toggle between a cat and a mouse. Screenshot from Wolfram Demonstrations Project<\/a>.<\/p><\/div>\n

On the theme of helpful animations, which teaching linear transformations, I found some really great demonstrations on Wolfram that let you transform a dog<\/a>, more specifically, a Scottish Terrier, by a personalized 2×2<\/i> matrix. You can stretch, flip, and shear<\/a> the Scottish Terrier by changing the values in the accompanying matrix. Somehow this is way more convincing than just drawing pictures and waving your hands around. The Wolfram Demonstrations Project<\/a> is packed with great demos for transforming vector spaces, and you can share your own. <\/p>\n

Finally, when I ask former math majors what most mystified them about Linear Algebra I almost always hear something about eigenvectors. It’s shocking how many students get in and out of Linear Algebra and have no intuitive idea what an eigenvector or eigenvalue are. And I’m not passing judgement here. When I took Linear Algebra as an undergraduate I was in the same boat! I knew how to compute them, but I had no idea what I was really looking at. <\/p>\n

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Not just a beautiful pirouette, but also a great example of a linear transformation with eigenvector e2<\/sub> and eigenvalue 1.<\/p><\/div>\n

Luckily, we have Steven Strogatz to the rescue with a most concise and intuitive explanation of eigenvectors and eigenvalues<\/a>. He compares a linear transformation of 3-dimensional space to snapshot of a dancer, arms outstretched spinning in a pirouette-like motion. Her arms (a vector in the x<\/i> direction) are moving, her gaze (a vector in the z<\/i> direction) is moving, but the leg she’s spinning on (a vector in the y<\/i> direction) stays fixed. This fixed vector is an eigenvector. And if she comes down of her pointed toes, then there is some element of scaling. This is an eigenvalue. <\/p>\n

Mathew Simonson, who wrote about his own “eigenightmares” for the AMS Grad Student blog<\/a>, proposes a spiral approach to pedagogy to combat those eigenfears. Students learn early on to express linear transformations as matrices, at this point they already can get some sense of eigen-type behavior just by acting on a simply figure in a vector space. Say, maybe, a nice little Scottish Terrier<\/a>. In this way, students can see that eigenvectors are happening, before formally knowing what they are. This puts the intuitive before the formal, which I like. <\/p>\n

Do you have any favorite online resources for teaching or understanding Linear Algebra? Let me know on Twitter @extremefriday<\/a>. <\/p>\n

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This semester I’m teaching Linear Algebra for the first time, so naturally, I am constantly on the prowl for all of the linear algebra resources the internet has to offer. To begin with, I’m using a free online textbook called … Continue reading →<\/span><\/a><\/p>\n

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