The Trig Hiding Inside the Factorials (And the Harmonic Numbers)

In this video, we build on my last two videos by exploring connections between the gamma function (the extended factorials), the digamma function (the extended harmonic numbers), and trigonometry. We derive Euler's Sine Product Formula, which we then use to prove the gamma and digamma functions' reflection formulas. Finally, we derive a related formula for calculating cotangent.
Watch my previous two videos here:
Extending the Harmonic Numbers to the Reals: • Extending the Harmonic...
How to Take the Factorial of Any Number: • How to Take the Factor...
An Elementary Proof of the Sine Product Formula:
www.researchgate.net/publicat...
The animations in this video were made with Manim: www.manim.community/
Music credits:
Fluidscape by Kevin MacLeod is licensed under a Creative Commons Attribution 4.0 license. creativecommons.org/licenses/...
Night Music by Kevin Macleod
Space Chillout by penguinmusic
river - Calm and Relaxing Piano Music by HarumachiMusic
Surrealism (Ambient Mix) by Andrewkn
... And a couple of my own songs:
/ the-fog
/ thanks-for-watching
Chapters:
00:00 Intro
0:43 Background and Notation
3:24 The Digamma-Cotangent Connection
5:09 The Gamma-Sine Connection
6:04 The Sine Product Formula
9:59 Proving the Gamma-Sine Connection
12:22 The value of (1/2)!
13:07 Proving the Digamma-Cotangent Connection
14:21 The True Logarithmic Derivative
15:52 An Infinite Sum for Cotangent
17:46 Final Thoughts

Пікірлер: 301

  • @LinesThatConnect
    @LinesThatConnect Жыл бұрын

    Some notes/clarifications that didn't make the cut: 2:29 It was a toss up between the Gamma function and the Pi function. (Pi(x) = Gamma(x + 1), so Pi(n) = n!) I much prefer the Pi function, but I've only seen it used a handful of times, while the Gamma function gets all the attention. I eventually decided it would be better to introduce people to the version they were more likely to see in the wild rather than using a notation they might never see again, so I went with the Gamma function. 5:21 You can get Greek letters and other LaTeX symbols in Desmos by typing them in a regular text editor (e.g. "\Gamma") and copy/pasting them in. That's how I got the Gamma there. 7:33 I put the pi's before the x's, even though the other order might at first be more natural -- (x - pi) instead of (pi - x). However, with (x - pi), the graph would be flipped upside down, which would be easily fixed by dividing by -pi instead of just pi. But I jumped straight to (pi - x) to save a bit of time. 15:00 I went back and forth between a few interpretations of the logarithmic derivative. If we had used the complex logarithm, everything would have just worked. But the background is a bit too complicated in my opinion. Another way would have been to use ln(|f(x)|), which still has all the nice logarithm properties, but it's less interesting and it doesn't extend to complex numbers very well. 16:57 Technically, we also need to show that the result converges uniformly to prove that this step is valid. But I'll leave that as an exercise for the viewer :)

  • @edomeindertsma6669

    @edomeindertsma6669

    Жыл бұрын

    Nice commutative diagram at the end.

  • @kyokajiro1808

    @kyokajiro1808

    Жыл бұрын

    5:26 i actually found this while messing around with this on desmos as well, i was looking into approximations of the factorial function and ended up trying 1/x! which i found to have a derivative everywhere (other than negative integers) so i messed around and got the sinc function (sin(x)/x) then shifted one of them over to get the sin function, i was so shocked by this discovery and was really confused but never really looked into it for some reason, really happy i finally got an explanation to it

  • @konradstec5737

    @konradstec5737

    10 ай бұрын

    I have a question for you. Do you have an email I can contact? Thanks!

  • @angeldude101

    @angeldude101

    10 ай бұрын

    With the Pi function, the reflection formula becomes 1/Π(x)Π(-x) = sin(πx)/πx. While the left-hand side is _way_ nicer than the Gamma version, the right hand side looks like it might not be... _at first glance._ Let's look back at the sine product formula: sin(x) = x ∏k=1→∞ (1 - (x/kπ)²)... wait. What's that x doing out front? Let's move it to the other side and see what happens: sin(x)/x = ∏k=1→∞ (1 - (x/kπ)²). The product now looks nicer, but the left-hand side... _That looks just like the reflection formula!_ sin(πx)/πx. Just to see what it's like, lets sub in πx to the product formula, since it looks like it should be able to cancel some thing: sin(πx)/πx = ∏k=1→∞ (1 - (πx/kπ)²) = ∏k=1→∞ (1 - (x/k)²). Beautiful. This function, sin(πx)/πx has its own name: the normalized sinc function. Often just written as sinc(x), or sometimes sinc(πx), where sinc(x) is instead the unnormalized version sin(x)/x, so you could call the normalized form "nsinc(x)" instead, but that's just something I made up. If you want a separate example of the sinc function (normalized or not) being useful on its own, _I dare you to take its Fourier Transform!_ If you haven't seen it before, you probably won't predict it.

  • @user-ne6km2oj8z

    @user-ne6km2oj8z

    8 ай бұрын

    Hi, can you pls find any connections between amount of isomers of alcanes for instance, i only found out that ratio between two adjacent numbers in line that does not include stereoisomers is suspiciously approaching exponent, and if including stereoisomers it is approaching π

  • @TonyboyDK
    @TonyboyDK Жыл бұрын

    This channel is one of the best things SoMe has spawned! As a mathematician myself, I really do appreciate the way you explain things, intuitive and visually beautiful but still with healthy respect for (and remarks about) the mathematical rigour! Love it!

  • @therobertguy2436

    @therobertguy2436

    Жыл бұрын

    What is SoMe? Edit: nvm! Summer of math exposition!

  • @unnikrishnanvr186

    @unnikrishnanvr186

    Жыл бұрын

    ​@@therobertguy2436 Its SoMething amazing

  • @Fire_Axus

    @Fire_Axus

    Ай бұрын

    your feelings are irrational

  • @Fire_Axus

    @Fire_Axus

    Ай бұрын

    some?

  • @mokhtarmougai5088
    @mokhtarmougai5088 Жыл бұрын

    The legend is back ❤

  • @krozjr5009
    @krozjr5009 Жыл бұрын

    I’d never FORMALLY heard of Euler’s product formula for sine before… but I’d actually wound up discovering it for myself. It’s always nice to know that I would’ve been a trailblazer if not for that meddling Euler.

  • @bakrom3734

    @bakrom3734

    Жыл бұрын

    Haha, I did the same thing but for the product representation of sin(x)/x. Feels good huh

  • @identicalgd2446

    @identicalgd2446

    5 ай бұрын

    Same

  • @wyboo2019

    @wyboo2019

    22 күн бұрын

    i have probably a dozen, completely-full 5-subject notebooks of math ive done, and i'm kinda hoping someone digs it up after im dead and theres something nontrivial in there lmao

  • @gumball6804
    @gumball6804 Жыл бұрын

    Math should always be presented like this; it's thoughtful, intruiguing and simply aweinspiring. The derivations lined up so nicely that it doesn't even feel abstract anymore.

  • @kikivoorburg
    @kikivoorburg Жыл бұрын

    Awesome! It's incredible how seemingly unrated mathematical ideas come together in unexpected ways!!

  • @amy53241
    @amy53241 Жыл бұрын

    I love that you pivot from frustrated that the digamma function is defined with x-1 to thankful when things canceled out xD

  • @user-kw2xs5rs2f
    @user-kw2xs5rs2f7 ай бұрын

    EVERY YEAR, HE UPLOADS A BANGER

  • @josh11735
    @josh11735 Жыл бұрын

    I love how this builds on your first two videos without being too overwhelming. Great job with the explanations (and I can’t wait to see what you cover next)! :)

  • @guslackner9270
    @guslackner9270 Жыл бұрын

    "And a couple of my own songs" Naturally. Amazing video; textbook quality. It is work like this that's going to turn Manim into the new LaTeX.

  • @eterty8335
    @eterty8335 Жыл бұрын

    I love these connections between weird mathematical functions that seem to come out of nowhere, so I can only imagine my excitement if I somehow managed to discover that reciprocal product being a sine wave after watching your previous video. This is amazing, thank you for these videos man.

  • @invincible9240
    @invincible9240 Жыл бұрын

    Bro your content in on an other level .Please don't stop uploading

  • @bunniesarecute3135
    @bunniesarecute3135 Жыл бұрын

    I love your channel! Not only do you present interesting connection, but the way you present those connections is really unique and ”simple”. What a great find :)

  • @ruferd
    @ruferd Жыл бұрын

    This is an absolute gem of a channel, I can't wait to have my mind blown even more in the next videos.

  • @scipio42
    @scipio42 Жыл бұрын

    I absolutely love these videos! It's crazy how fantastically intuitive the explanation and presentation are

  • @omarelzeki_
    @omarelzeki_ Жыл бұрын

    I genuinely hope this channel can grow as big as possible as quick as possible. Wonderful content❤️❤️

  • @edmundwoolliams1240
    @edmundwoolliams1240 Жыл бұрын

    This is incredible! Better than 3b1b, because it’s straight to the point. I’ve been waiting for you to post a video for ages. Keep up the good content!

  • @DorkOrc
    @DorkOrc Жыл бұрын

    I really appreciate the subtitling, the sound design, and the visuals. I never had to adjust the volume, the music never overpowered your voice, the animations were very easy to follow, and the subtitles were spot on. Oh and the maths was kinda mindblowing!

  • @alperxyz
    @alperxyz Жыл бұрын

    It's nice to see elegant connections between different math concepts. I like your videos, hope they continue.

  • @NoahK216
    @NoahK216 Жыл бұрын

    Very excited for more content! I loved the last two

  • @jimschneider799
    @jimschneider799 Жыл бұрын

    I've always heard that Euler's identity is "the most beautiful formula in mathematics", but after watching this video, I have to say I think the digamma reflection formula is a serious contender. It's one of those surprising connections that's obvious in retrospect that make me love math. Thank you for such a great video.

  • @angeldude101

    @angeldude101

    10 ай бұрын

    Since the gamma and digamma functions were both offset from their discrete counterparts, I decided to check the reflection formula for the extended harmonics and got H(x) - H(-x) = 1/x - πcot(πx). The left-hand side is quite a bit nicer than the digamma version, but the right hand side not so much given that extra 1/x.

  • @kono152
    @kono152 Жыл бұрын

    I cannot express the happiness that I felt seeing you posted a new video! I'm so excited to watch this

  • @satyadas4519
    @satyadas4519 Жыл бұрын

    I watched 3 videos of yours back to back and I am still not bored. Great work. Thank you for making these videos.

  • @kudzufarmer5431
    @kudzufarmer5431 Жыл бұрын

    I need these videos more than once a year-ish. I also think it would be cool to explore the gamma constant more in a future video, but you do you. I'm just enjoying the ride.

  • @davidcotham1939
    @davidcotham1939 Жыл бұрын

    Keep up the great videos! Mark my words... You are going to be one of the greats! I'm calling it now.

  • @proxagonal5954
    @proxagonal5954 Жыл бұрын

    Your videos really are great. You explain things very well, and at a very good pace. You always conclude the video reaching very satisfying and unexpected results. I love it.

  • @daniellewilson8527
    @daniellewilson852710 ай бұрын

    I love your channel, I like that you derive the formulae. I also like that you use white text on black background, it doesn’t hurt my eyes like the inverse does. Also I like that you center the equation you’re working on, it makes it easy to see even on a small screen. Your voice is nice and I like that you say what the names of the symbols are as you’re showing them

  • @lexinwonderland5741
    @lexinwonderland5741 Жыл бұрын

    Dude, i'm RE watching this two weeks later, after having watched it multiple times in a row as soon as it came out (subscribed with notifications hehe) -- it made perfect sense on the first watch because your explanation was so smooth and intuitive, but because there are SO MANY HIDDEN LAYERS TO IT! I've been doing this stuff for at least a decade.... and I *still* learned things from this video, and then learned more every time I re-watch to unearth more. Please please please, keep it up!!

  • @sergiogarciacastro9103
    @sergiogarciacastro9103 Жыл бұрын

    This is simply beautiful. An awesome series of videos, great job!!

  • @bennyloodts5497
    @bennyloodts5497 Жыл бұрын

    With simple small steps.... climbing the mountain, is only possible with an excellent guide, The view is breathtaking! Thank you so much!

  • @lexus_bkl
    @lexus_bkl Жыл бұрын

    Finally a new video after months! I really like your videos, please be consistent on your channel.

  • @robharwood3538
    @robharwood3538 Жыл бұрын

    Awesome! You've managed to connect so many dots in my head that I'm finally able to grasp how all these concepts fit together. Thank you! Would love to see something on how to calculate the incomplete gamma function (integral formula for gamma from 0 to x rather than 0 to infinity). Key applications: Gamma probability distribution, machine learning, and many more.

  • @AlbertTheGamer-gk7sn
    @AlbertTheGamer-gk7sn Жыл бұрын

    These functions are important when dealing with fractional calculus, such as finding the half integral of 1, which is 2sqrt(x/pi). The half integral of 0 is surprisingly 1/sqrt(pi*x).

  • @ffc1a28c7

    @ffc1a28c7

    Жыл бұрын

    Note that this is only really important because we have defined it to be that way. Cauchy's integral formula is true for analytic functions, and asserts that we can infinitely differtiate (and integrate) any analytic function. It happens to use factorials because of its iterative nature, which lends to using the gamma function. Fractional derivatives and integrals aren't some innate thing (though they have their uses), but are a product of some fancy magic.

  • @HomieSeal
    @HomieSeal Жыл бұрын

    Dude I was literally JUST thinking about your channel LMAO Glad to see a new video, keep up the good work :)

  • @elijah_9392
    @elijah_9392 Жыл бұрын

    One of the fastest clicks of my life. I'm glad to see you upload again!

  • @GurshanParmar
    @GurshanParmar Жыл бұрын

    Omg I absolutely love to see new uploads from you!

  • @General12th
    @General12th Жыл бұрын

    Hi Lines! I love the captions! That's very good even for people who aren't hard of hearing.

  • @TheJara123
    @TheJara123 Жыл бұрын

    Terrific!! Great videos man, please post more often then three videos in one year...please!!

  • @Ooipstronaughtilus10000
    @Ooipstronaughtilus10000 Жыл бұрын

    I was putting off watching this video, until I saw your channel name under the title! I’m glad you’re still posting!

  • @shreddaification
    @shreddaification Жыл бұрын

    Incredible, may you have a long and prosperous career!

  • @t1t0uan
    @t1t0uan Жыл бұрын

    as usual, very interesting and beautiful video, can't wait to see the nexts !

  • @nianyiwang
    @nianyiwang Жыл бұрын

    it has been one freaking year for this channel to make its 3rd video but the quality is gold

  • @LeetMath
    @LeetMath Жыл бұрын

    love your videos glad you’ve kept making more

  • @Patrik2569
    @Patrik2569 Жыл бұрын

    first time seing this channel, but i must sub. Great job with explanations. I loved how you stoped at some points, and explained the things that weren't so obvious.

  • @michaelgolub2019
    @michaelgolub2019 Жыл бұрын

    I have watched all the tree videos and though I was aware of almost every point, I was deeply impressed by the constructive approach of the author. The interconnection between various functions looks very pronounced here. Moreover, the formulae may be used not only for some analysis but also for computational reasons (at least to estimate rate of convergence).

  • @atheybengala5720
    @atheybengala5720 Жыл бұрын

    HOLY ANOTHER VIDEO, THIS MUST BE GREAT!

  • @DrPhipster
    @DrPhipster2 ай бұрын

    This channel is simply fantastic! ❤

  • @jespermikkelsen7553
    @jespermikkelsen7553 Жыл бұрын

    Bro, you did it again. Amazing video 🙏

  • @AB-gf4ue
    @AB-gf4ue Жыл бұрын

    Yes, king! We love lines that connect!

  • @_unkown8652
    @_unkown8652 Жыл бұрын

    I’ve been waiting for this video for so long!

  • @clover7359
    @clover7359 Жыл бұрын

    Fascinating math and great animations too.

  • @mohammed333suliman
    @mohammed333suliman Жыл бұрын

    Great stuff, Welcome back

  • @vitalysarmaev
    @vitalysarmaev Жыл бұрын

    You are a genius of explanation! ❤

  • @skeet1441
    @skeet144111 ай бұрын

    your videos are so well made and entertaining!! ❤❤

  • @ahmetemiruludag
    @ahmetemiruludag Жыл бұрын

    Subscribed after the factorial video, I can say that Im not dissapointed! Well done

  • @dominicellis1867
    @dominicellis1867 Жыл бұрын

    I love Fourier analysis. It connects music and number theory, trig, analytic continuation, and differential equations. I’m working on a Fourier transform that can take a musical phrase and output a mathematical equation.

  • @PMA_ReginaldBoscoG

    @PMA_ReginaldBoscoG

    Жыл бұрын

    If you can, will you create an open source software for such transformation ?

  • @dominicellis1867

    @dominicellis1867

    Жыл бұрын

    @@PMA_ReginaldBoscoG of course. I plan on creating the transform in both directions so you can translate between math and music. If you assign a value theta and phi for every pitch and rhythm of a given phrase, then the magnitude of the dot product of the theta and phi vectors equals the resultant amplitudes. Thus, integrating across these dot products should output the phrase structure of that measure. Thus, you can take in a musical phrase and output an integral across an inner product space or take an inner product space and output a musical phrase. I use angles for pitches and rhythms because music is cyclical and repetitive.

  • @fengshengqin6993
    @fengshengqin6993 Жыл бұрын

    Your video is extremely enjoyable ! Thanks!!

  • @MegaMuffinManX
    @MegaMuffinManX11 ай бұрын

    This was great! Thanks for posting

  • @elyepes19
    @elyepes19 Жыл бұрын

    Bravo and applause! Thank you so much 👍🏽

  • @martiribapons
    @martiribapons Жыл бұрын

    Thank you for this amazing video! Your channel is amazing!

  • @mikoajwrobel4561
    @mikoajwrobel4561 Жыл бұрын

    Maybe it's because of me being in high school - you said it in your first video thats your vides are targeting ones like me - but I find your videos incredibly interesting, finally telling me very desired answers for my "mathy" questions. I would be grateful if you'd make more of them😀❤. I am sure that you are going to make a big and happy community of viewers🤩

  • @amiralirahimi2219
    @amiralirahimi22192 ай бұрын

    Amazing. It was one of the most amazing video I've ever seen

  • @gczhu5125
    @gczhu5125 Жыл бұрын

    Absolutely wonderful!!! I said this many times before and I will have to say it again. You belong to the BEST graduate school. You should be doing your PhD at MIT, or Harvard, or Cambridge, or anywhere you want. 🎉🎉🎉

  • @CarmenLC
    @CarmenLC Жыл бұрын

    so true, i can never remember if Γ(x) = (x-1)! or Γ(x-1) = x!

  • @CarmenLC

    @CarmenLC

    Жыл бұрын

    but Γ(x+1) = xΓ(x) 😅

  • @_ajmah__
    @_ajmah__ Жыл бұрын

    Underrated channel

  • @YTomS
    @YTomS Жыл бұрын

    Incredible work, as always :).

  • @vikramdorbala5198
    @vikramdorbala5198 Жыл бұрын

    Thankyou so much for making these videos

  • @TheJoyLoveShow
    @TheJoyLoveShow22 күн бұрын

    Omg this is the Best!! Thank you!!😊

  • @marwann1
    @marwann1 Жыл бұрын

    Well put together, you earned a sub ❤

  • @fellipetoffolo4226
    @fellipetoffolo4226 Жыл бұрын

    I was waiting for this, very nice !!!!

  • @r3birth
    @r3birth Жыл бұрын

    these videos are amazing! keep them coming :)

  • @karans6762
    @karans6762 Жыл бұрын

    YES HE"S BACK I LOVE YOU

  • @abutorabrahman4114
    @abutorabrahman4114 Жыл бұрын

    Wow!!So much intuitive.. ❤❤❤

  • @ApolloGorillaTag
    @ApolloGorillaTag11 ай бұрын

    I love these videos!!

  • @leonard-he6hd
    @leonard-he6hd Жыл бұрын

    BEST THANK ZOU FOR DOING MY DAY

  • @shcottam
    @shcottam Жыл бұрын

    This was actually like a cool video and it was about math!? Nice job dude

  • @Ipad-yg1tl
    @Ipad-yg1tl Жыл бұрын

    Let’s goooo he’s back, had me worried there for a minute

  • @frtzkng
    @frtzkng4 ай бұрын

    For an even neater representation of those sine terms in the denominator, one can use *secant* (sec) and *cosecant* (csc). They're defined as sec(x) = 1/cos(x) and csc(x) = 1/sin(x), which turns those fractions in the reflection formula into products.

  • @Dessoxyn
    @Dessoxyn Жыл бұрын

    This is an excellent video, it really utilizes the strengths and abilities of the medium (video) in order to more clearly explain things. The (literal) video, editing, and narration all reinforce each other in a way that is almost Kubrick-like :) Just add a Wendy Carlos soundtrack and you're basically there. Did I understand the video? Nope. Did I learn things? Yup. Did I learn a lot more than I expected? F Yeah! There are certain fields where I find it extremely useful to dip my toe in the deep end or "get comfortable with being uncomfortable." I learn a lot in the present, I'll sometimes have a drive to "catch up" on certain subtopics, and my brain has already started figuring out the "hard" stuff, or it's laying a foundation, whatever metaphor you want to use. Thanks!

  • @majdsaleh_
    @majdsaleh_ Жыл бұрын

    Youe videos are absolutely great man 👌🏻 But just don't make me wait another 8 months for the new one 😊🤝🏻

  • @ismahelo
    @ismahelo Жыл бұрын

    im crying this is beautiful

  • @alexander_elektronik
    @alexander_elektronik3 ай бұрын

    amazing topic!

  • @usernameisamyth
    @usernameisamyth Жыл бұрын

    great stuff, man!!!

  • @qbish_
    @qbish_5 ай бұрын

    Make more vids bro😢

  • @RobinHillyard
    @RobinHillyard Жыл бұрын

    This is really good!

  • @EPMTUNES
    @EPMTUNES Жыл бұрын

    Your videos are always a joy

  • @ErdemtugsC
    @ErdemtugsC5 күн бұрын

    Best things in math: 1-“Let’s put a box around it” 2- cleaning up and simplifying a mess

  • @kalleanka5043
    @kalleanka5043 Жыл бұрын

    Thank you 🎉🎉😊

  • @sentientteapot6499
    @sentientteapot6499 Жыл бұрын

    RETURN OF THE KING

  • @ossigaming8413
    @ossigaming84139 ай бұрын

    I'd love to see the full proof made by you

  • @trentyoung274
    @trentyoung274 Жыл бұрын

    Keep up the great work!

  • @kierkegaard54
    @kierkegaard5410 ай бұрын

    this is amost as good as 3blue1brown, really nice work man

  • @shmuelalexis9836
    @shmuelalexis9836 Жыл бұрын

    Beautiful.

  • @iHATEbigots666
    @iHATEbigots666 Жыл бұрын

    This is helping me on my sick day

  • @shahars3134
    @shahars3134 Жыл бұрын

    Awesome video! Keep up the good work

  • @raagamparmar5602
    @raagamparmar5602 Жыл бұрын

    This is great! Awesome!

  • @Nazario-Tech
    @Nazario-Tech Жыл бұрын

    Your videos are amazing!

  • @aze4308
    @aze4308 Жыл бұрын

    great video, as always

  • @avunz125
    @avunz125 Жыл бұрын

    Math is so elegant. Your explanations and animations truly highlight the beauty of maths. Thank you very much. Looking forward to your future videos as much as I was waiting for this one :)

  • @tomkerruish2982
    @tomkerruish2982 Жыл бұрын

    You likely know this already, but your formula for cotangent is very reminiscent of Eisenstein's approach to trigonometry. I first encountered it in Remmert's magnificent text Theory of Complex Functions, which is quite possibly the finest textbook I've ever read.