diff --git a/posts/math/chebyshev/1/index.qmd b/posts/math/chebyshev/1/index.qmd index 0da1147..e590e6e 100644 --- a/posts/math/chebyshev/1/index.qmd +++ b/posts/math/chebyshev/1/index.qmd @@ -259,18 +259,18 @@ $$ = (z^2 - z - 1) (z^2 + z - 1) $$ -According to how we derived this series, when $z = 2\cos(\theta)$, the roots of this polynomial +According to how we derived this polynomial, when $z = 2\cos(\theta)$, the roots correspond to when $\sin(5\theta) / \sin(\theta) = 0$. This relation itself is true when $\theta = \pi / 5$, since $\sin(5 \pi / 5) = 0$. One of the factors must therefore be the minimal polynomial of $2\cos(\pi / 5 )$. -The former happens to be correct correct, since $2\cos( \pi / 5 ) = \varphi$, the golden ratio. +The former happens to be the correct choice, since $2\cos( \pi / 5 ) = \varphi$, the golden ratio. Note that the second factor is the first evaluated at -*z*. #### Heptagons -An example of where constructability fails is for $2\cos( \pi / 7 )$. +An example where constructability fails is for $2\cos( \pi / 7 )$. $$ \begin{align*} @@ -289,7 +289,8 @@ But there are no (nondegenerate) cubics that one can produce via compass and str #### Decagons -One might think the same of $2\cos(\pi /10 )$ +One might think the same of $2\cos(\pi /10 )$. +The relevant polynomial is: $$ \begin{align*} @@ -300,16 +301,17 @@ $$ \end{align*} $$ -This expression also contains the polynomials for $2\cos( \pi / 5 )$. -This is because a regular decagon would contain two disjoint regular pentagons, - produced by connecting every other vertex. +This expression also contains the polynomials for $2\cos( \pi / 5 )$, + since connecting every other vertex of a regular decagon produces + two disjoint regular pentagons. ![   ](./decagon_divisible.png) The polynomial which actually corresponds to $2\cos( \pi / 10 )$ is the quartic, - which seems to suggest that it will require a fourth root and somehow decagons are not constructible. + which seems to suggest that it will require a fourth root and imply that decagons + are somehow not constructible. However, it can be solved by completing the square... $$ @@ -496,12 +498,12 @@ $$ If $z = 1$, these two generating functions are equal. The same can be said for $z = 2$ with the generating function of the Pell numbers, and so on for higher recurrences (corresponding to metallic means) for higher integral *z*. +In fact, it is possible to find the Fibonacci numbers (Pell numbers, etc.) + in Pascal's triangle, which you can read more about + [here](http://users.dimi.uniud.it/~giacomo.dellariccia/Glossary/Pascal/Koshy2011.pdf). In terms of the Chebyshev polynomials, this series manipulation removes the alternation in the coefficients of $U_n$, restoring Pascal's triangle to its nonalternating form. -Related to the previous point, it is possible to find the Fibonacci numbers (Pell numbers, etc.) - in Pascal's triangle, which you can read more about - [here](http://users.dimi.uniud.it/~giacomo.dellariccia/Glossary/Pascal/Koshy2011.pdf). Manipulating the Series @@ -674,6 +676,7 @@ Though they were present in the earlier Chebyshev table, ```{python} #| echo: false +#| label: tbl-cosinepolynomials #| classes: plain def poly_to_rising_power_list(poly, var): diff --git a/posts/math/stereo/2/index.qmd b/posts/math/stereo/2/index.qmd index 6ac7760..964b99b 100644 --- a/posts/math/stereo/2/index.qmd +++ b/posts/math/stereo/2/index.qmd @@ -190,7 +190,7 @@ For comparison, the complex exponential (as it is a parallel construction) has a Since the exponential function is its own derivative, the expression acquires an imaginary coefficient through the chain rule. -[^1]: This is forgoing the fact that complex derivatives require more care than their real counterparts. +[^1]: This is forgoing the extra care that complex derivatives require beyond their real counterparts. It matters slightly less in this case since this function is complex-valued, but has a real parameter. $$ @@ -249,12 +249,12 @@ $$ = 2\pi i $$ -In this example, Γ is a counterclockwise curve parametrized by γ which loops once around +In this example, *Γ* is a counterclockwise curve parametrized by *γ* which loops once around the pole at *z* = 0. More loops will scale this by a factor according to the number of loops. Normally this equality is demonstrated with the complex exponential, but will $o_1$ work just as well? -If Γ is the unit circle, the integral is: +If *Γ* is the unit circle, the integral is: $$ \oint_\Gamma {1 \over z} dz @@ -281,9 +281,9 @@ $$ \end{gather*} $$ -It is certainly possible to perform these contour integrals along straight lines; - in fact, integrating along lines from 1 to *i* to -1 to -*i* deals with a - similar integral involving arctangent. +It is certainly possible to perform these contour integrals along straight lines + in the complex plane; in fact, making *Γ* a diamond-shaped contour from + 1 to *i* to -1 to -*i* produces a similar integral involving arctangent. However, the best one can do to construct more loops with lines is to count each line multiple times, which isn't extraordinarily convincing. @@ -304,7 +304,7 @@ $$ $$ This has an additional benefit: using the series form of $1 / (1 + t^2)$ and integrating, - one obtains the series form of the arctangent. + one obtains [the series form of arctangent](https://en.wikipedia.org/wiki/Arctangent_series). This series converges for $-1 \le t \le 1$, which happens to match the bounds of integration. The convergence of this series is fairly important, since it is tied to formulas for π, in particular [Leibniz's formula](https://en.wikipedia.org/wiki/Leibniz_formula_for_%CF%80).