Wikipedia:Reference desk/Archives/Mathematics/2023 January 28

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January 28

Interesting trigonometric identities...but why???

So this morning I "rediscovered" something I had stumbled on a few years back:

First, define $A=\tan ^{-1}(1)$, $B=\tan ^{-1}(2)$, and $C=\tan ^{-1}(3)$. Taken separately, they seem to be nothing more than a trio of nondescript irrational numbers (0.785398163397..., 1.107148717794..., and 1.249045772398..., respectively). In certain combinations however, the result is rather surprising. Notably, $A+B+C=\pi$ and ${\frac {B+C}{A}}=3$.

Very nice, but what is really going on here? I can't for the life of me remember how exactly I derived these identities in the first place. (Ah the joys of old age!) I do remember that $\pi =4\tan ^{-1}(1)$, but that doesn't really seem to help much here. Earl of Arundel (talk) 15:14, 28 January 2023 (UTC)

At the first glance, A is 45 degree (or 1/4 pi rad). At this point both equalities appear equivalent: B+C must be 3A to make the sum of three equal 4A. --CiaPan (talk) 15:31, 28 January 2023 (UTC)

Ah yes, simple algebra. Well thanks for helping to clear up the old fog. :) (Although I do wish I could remember how I found B and C to begin with!) Earl of Arundel (talk) 16:24, 28 January 2023 (UTC)

Explanation how arcus-tangents of 1, 2, and 3 add up to pi (a straight angle at O)

Make a drawing. Get a plane with Cartesian coordinates. Mark points O(0,0), P(1,0), Q(1,1), R(-1,3), and S(-1,0). Consider triangles OPQ, OQR, and ORS. Identify right angles in those triangles and then identify tangents of the three angles at O. Good luck! CiaPan (talk) 16:38, 28 January 2023 (UTC)

I see. So really this could be generalized to produce similar identities. That's really interesting. And such a simple geometric construction too. Well thank you for the enlightening perspective. Cheers! Earl of Arundel (talk) 17:17, 28 January 2023 (UTC)

Or, somewhat less exciting, by using the identity for the tangent of a sum. Let $a,b,c\,$ be a triple satisfying $a+b+c=abc,$ and define $A=\tan ^{-1}a,~B=\tan ^{-1}b,~C=\tan ^{-1}c.$ Then $A+B+C=\pi .$

For example, ${\frac {7}{6}}+{\frac {4}{3}}+{\frac {9}{2}}={\frac {7}{6}}\cdot {\frac {4}{3}}\cdot {\frac {9}{2}}=7,$ and

${\begin{array}{ccl}\tan ^{-1}{\dfrac {7}{6}}&{=}&0.862170...\\\tan ^{-1}{\dfrac {4}{3}}&{=}&0.927295....\\\tan ^{-1}{\dfrac {9}{2}}&{=}&1.352127...\\&&{-}\!\!{-}\!{-}\!{-}\!{-}\!{-}\!{-}\!\!{-}~+\\&&3.141592...\end{array}}$

 --Lambiam 18:09, 28 January 2023 (UTC)

Nice! So {1, 2, 3} are the only possible integer solutions. It isn't immediately obvious how you came up with those particular values (should be an interesting after-lunch exercise anyhow). Earl of Arundel (talk) 18:48, 28 January 2023 (UTC)

Pick $a$ and $b$ such that $ab\neq 1$ and set $c={\frac {a+b}{ab-1}}.$ With some trial and error you'll find dozens of relatively simple rational triples, some much simpler than this example. For example, $a=1,b=5$ gives you $c={\tfrac {3}{2}}.$  --Lambiam 19:29, 28 January 2023 (UTC)

Neat! So in fact there are many, many such solutions. Well that's even better than the original question. What an uncanny talent for maths you have, Lambiam. I honestly envy your abilities! Earl of Arundel (talk) 23:28, 28 January 2023 (UTC)

You need not even mention any transcendental function. (6+7i)·(3+4i)·(2+9i) = -425, a negative real number. —Tamfang (talk) 00:14, 29 January 2023 (UTC)

@Earl of Arundel: Please see the image I added (I made it in https://www.geogebra.org/classic). --CiaPan (talk) 19:25, 28 January 2023 (UTC)

Thank you! The rough sketch I put together in MS-Paint wasn't nearly as helpful. Earl of Arundel (talk) 23:34, 28 January 2023 (UTC)

Another fun one is ${\tfrac {1}{2}}\varpi =2\operatorname {arcsl} {\tfrac {1}{2}}+\operatorname {arcsl} {\tfrac {7}{23}}$ where $\varpi$ is the lemniscate constant and $\operatorname {arcsl}$ is the inverse lemniscate sine.

But for more about the circular ones, see Machin-like_formula. –jacobolus (t) 22:31, 28 January 2023 (UTC)

Wow, I've never even heard of a "lemniscate". So this arcsl function shares very similar constructions as the atan function. Fascinating. And these Machin-like formulas are very interesting too. Especially how they (more or less) relate to the complex plane. Well thank you for the wonderful links. Great stuff! Earl of Arundel (talk) 00:02, 29 January 2023 (UTC)

The lemniscate of Bernoulli ${\textstyle r^{2}=\cos 2\theta }$ is one kind of quartic analog of the unit-diameter circle $r=\cos \theta .$ These are both "clovers", named after the curve ${\textstyle r^{3/2}=\cos {\tfrac {3}{2}}\theta .}$ (See Cox & Shurman, 2005.) A different quartic analog of the circle is the quartic Fermat curve ${\textstyle x^{4}+y^{4}=1,}$ analogous to the unit circle. Both the lemniscate and the Fermat quartic are closely related to the lemniscate constant $\varpi ,$ the way the circle is related to $\pi .$ In the complex plane, these are related to the lattice of Gaussian integers the way the circle and $\pi$ are related to the ordinary integers. There is also a cubic analog ${\textstyle x^{3}+y^{3}=1}$ which is related to the Dixon elliptic functions, and involves yet another $\pi$-analogous constant, and in the complex plane, the Eisenstein integers. –jacobolus (t) 02:46, 30 January 2023 (UTC)

I honestly don't quite know what to make of all that! It almost seems like an entirely exotic field of maths. Well it's going to take some time to sink in. In any case, thank you for opening up my horizons. On a side note, I don't suppose this lemniscate constant has a more simple closed-form expression (ie, sans the Riemann zeta function term)? Earl of Arundel (talk) 15:08, 30 January 2023 (UTC)

The easiest one to write down is ${\textstyle \varpi ={\tfrac {1}{2}}\mathrm {B} {\bigl (}{\tfrac {1}{4}},{\tfrac {1}{2}}{\bigr )}}$ in terms of the Beta function which is a kind of continuous analog of binomial coefficients. Personally I think studying the lemniscate functions (related to the hyperbola-analogous curve ${\textstyle x^{4}-y^{4}=1}$) is a bit less obvious for me than studying the Fermat curve ${\textstyle x^{4}+y^{4}=1,}$ which comes from a ${\textstyle {\tfrac {1}{8}}}$-turn rotation in the complex plane so winds up involving constants related to the lemniscate constant by square roots of two (the diagonal of the square), e.g. the constant ${\textstyle {\tfrac {1}{4}}\mathrm {B} {\bigl (}{\tfrac {1}{4}},{\tfrac {1}{4}}{\bigr )}={\tfrac {1}{\sqrt {2}}}\varpi }$ is the area under the quartic Fermat curve in the first quadrant ${\textstyle x>0,\,y>0,}$ and in general ${\textstyle {\tfrac {1}{n}}\mathrm {B} {\bigl (}{\tfrac {1}{n}},{\tfrac {1}{n}}{\bigr )},}$ is the area under the $n$th Fermat curve in the first quadrant, so the analog in the circle case is ${\textstyle {\tfrac {1}{2}}\pi ={\tfrac {1}{2}}\mathrm {B} {\bigl (}{\tfrac {1}{2}},{\tfrac {1}{2}}{\bigr )}.}$ –jacobolus (t) 17:04, 30 January 2023 (UTC)

This is an old puzzle just made more obscure. See [1] for an example of it on the web together with a solution. NadVolum (talk) 20:04, 28 January 2023 (UTC)

Awesome, thanks for the link! Earl of Arundel (talk) 23:37, 28 January 2023 (UTC)

While the OP enquired about $\tan ^{{-}1}1+\tan ^{{-}1}2+\tan ^{{-}1}3=\pi ,$ this puzzle establishes that $\cot ^{{-}1}1+\cot ^{{-}1}2+\cot ^{{-}1}3={\frac {\pi }{2}}.$ Of course, using $\tan ^{{-}1}a+\cot ^{{-}1}a={\frac {\pi }{2}},$ these two problems are interconvertible, but they do not seem entirely equivalent when viewed as puzzles.  --Lambiam 09:52, 29 January 2023 (UTC)

Another way to express this is $1\oplus {\tfrac {1}{2}}\oplus {\tfrac {1}{3}}=\infty$ $\implies {\tfrac {1}{2}}\oplus {\tfrac {1}{3}}=\infty \ominus 1=1$ where ${\textstyle a\oplus b={\frac {a+b}{1-ab}}}$ is the tangent sum operation. For a fun geometric construction using an auxiliary hyperbola, see Kocik (2012).

–jacobolus (t) 17:49, 31 January 2023 (UTC)

inside-out stars

Consider a red p/q star inscribed in a blue regular p-gon. If you rearrange the vertices so that the red figure is convex, the blue edges become a p/r star. Can you easily express r as a function of p,q? —Tamfang (talk) 23:44, 28 January 2023 (UTC)

It is the modular multiplicative inverse of q with respect to p. If r comes out larger than p / 2, the traversal along the edges proceeds in the opposite sense of that of the {p / q } star; replace r by p − r to get the standard Schläfli symbol for this regular star polygon. Interestingly, some are self-dual, like {8/3} → {8/3} and {12/5} → {12/5}. This happens when p｜(q² − 1). Some others are self-dual after edge reversal, like {5/2} → {5/−2} and {13/5} → {13/−5}. This happens when p｜(q² + 1).  --Lambiam 00:17, 29 January 2023 (UTC)

All but one of your examples are Fibonacci stars; a quick experiment suggests that all of these are self-dual in one sense or the other (alternating). —Tamfang (talk) 18:13, 1 February 2023 (UTC)