Half derivative ln(x)

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  • Опубліковано 15 гру 2024

КОМЕНТАРІ • 132

  • @granhermon2
    @granhermon2 6 років тому +112

    Now half-differentiate again to check it's right :P

  • @albertodelaraza4475
    @albertodelaraza4475 6 років тому +27

    This problem requires a solid understanding of both Differential and Integral Single Variable Calculus to solve. Even if the definition of a half derivative is given, along with the gamma function value, any student that can complete the solution has definitely demonstrated mastery of the subject. Just go ahead and give them an A for the course.

  • @leonardromano1491
    @leonardromano1491 6 років тому +66

    Do fractional derivatives also have things like the product rule or the chen lu? Also I'm triggered by ln(x)+ln(4) because ln(4x) is much more beautiful!

    • @rezlana
      @rezlana 6 років тому +12

      Leonard Romano A while back I played around with half derivatives on a heuristic non-rigorous level and I noticed that because the higher order derivatives of a product of functions followed the binomial theorem, you can use fractional power generalizations of the binomial theorem as well. I can't seem to find where I wrote anything about it down, but iirc it seemed like it converged to reasonable functions, but I was only using really simple products that I was able to take half derivatives of themselves, like f(x)=x^2=x*x or f(x)=sin(x)cos(x)=sin(2x)/2

    • @mathevengers1131
      @mathevengers1131 2 роки тому

      I am also triggered by that thing.

  • @TZPlayer
    @TZPlayer 6 років тому +3

    Wait, what, literally yesterday I was thinking about this, I was just about to comment in Dr. Peyam's latest video to do this problem, I can't express how overjoyed I am!
    Ps: Hi from Brazil

  • @MF-lg8mt
    @MF-lg8mt 6 років тому +10

    Riemann-Liouville derivative. Use now Caputo Derivative and compare results. Great video. Best regards from México!

  • @silasrodrigues1446
    @silasrodrigues1446 6 років тому +2

    Man! That was really amazing! I lost my breath in some parts but I think I got it. I'll copy the initial problem and try to solve it on my own tomorrow. Then I'll start to play picking up other functions to half differentiate! Thank you very much Dr. P!

  • @williamadams137
    @williamadams137 5 років тому +1

    👍👍amazing work man!!! Best half derivative video

  • @Materialismodialecticohoy
    @Materialismodialecticohoy 6 років тому +24

    The definition of the fractional derivative is very similar to the complex Cauchy theorem.

    • @drpeyam
      @drpeyam  6 років тому +18

      Wow, it is! Didn’t realize that!

    • @112BALAGE112
      @112BALAGE112 6 років тому +4

      Materialismo Dialéctico Hoy That can't be a coincidence.

    • @LilithLuz2
      @LilithLuz2 2 роки тому +2

      @@112BALAGE112 It isn't, the fractional derivative formula is derived from it

  • @kharnakcrux2650
    @kharnakcrux2650 Рік тому

    24:45 i put ln(4x)
    I graph this stuff in GraffEQ imagine an animation, if alpha varies smoothly. you'll see that 0 point whip, up & down, while the Negative side makes weird spirals/irrational discontinuous. That's what i call a "Derivative spectrum". I would naiively let alpha be any Complex C

  • @marcelorogel9465
    @marcelorogel9465 5 років тому

    Greetings from Chile, you are the best mathTuber

  • @kamilbizon8317
    @kamilbizon8317 6 років тому +8

    These half derivatives are amazing, i would love to see even more complex problems

  • @Koisheep
    @Koisheep 6 років тому +4

    Dr Peyam, may I ask if differentiability on an open interval is a sufficient condition to guarantee the existence of a half-derivative?

    • @drpeyam
      @drpeyam  6 років тому +2

      We don’t even need differentiability, I feel just some sort of continuity is enough!

    • @michalbotor
      @michalbotor 6 років тому +3

      .. so i was casually looking for this answer myself, and i came across the fractional derivative of the weierstrass function.
      and now i can't sleep.
      help?

  • @BabyXGlitz
    @BabyXGlitz 5 років тому +1

    بارك الله فيك يا دكتور پايام

  • @michalbotor
    @michalbotor 6 років тому +10

    dr peyam, could you perhaps solve the equation D^a f = f, where 0 < a < 1 next?
    i believe, that it is a naturally coming to mind question to ask what is an analogue of an exponential function for this "fractional" case and quite possibly of great practical importance too. however as simple as it initially seemed to be to solve, it kind of quickly overgrown me, and i got stuck. yelp? ;p

    • @drpeyam
      @drpeyam  6 років тому +7

      Wow, great question :) I feel it would be something like C e^bx, for some b in terms of a

    • @michalbotor
      @michalbotor 6 років тому +1

      thank you. ;) whatever it is, i hope it will be as astounding, revealing and beautiful as the "fractional" derivatives of the trigonometic functions were.

    • @NAMEhzj
      @NAMEhzj 6 років тому +1

      This is actually not that hard: As long as a is a rational number: a = n/m for n,m in Nat, iirc it follows from the definition of D^a , that (D^a)^m f = D^(a*m) f = D^(n/m *m) f = D^n f. So, if D^a f = f then iterating m times gives (D^a)^m f = f, so D^n f = f. but for this Differential equation we certainly know the unique solution f(x) = c*e^x for some constant c. So the awnser has to be this. (or no awnser exists, which would be sad). Then, if we assume that the D to the something operator is continuous with respect to the power (which should follow from the formula) , the real case is also clear, since rationals are dense in R. :)

    • @michalbotor
      @michalbotor 6 років тому

      thank you kindly for giving it a go. however this cannot be the answer since it doesn't work for example for half-derivative of e^x. at least according to this big-headed genius:
      www.wolframalpha.com/input/?i=1%2Fgamma(1%2F2)+*+derivative+with+respect+to+x+of+integral+from+0+to+x+of+e%5Et%2F(x-t)%5E(1%2F2)+dt

    • @NAMEhzj
      @NAMEhzj 6 років тому

      Well, it could just be that Wolframalpha cant compute that integral. (since its pretty complicated after all) Or maybe the equation doesnt have any solutions whatsoever. What i did was show that if there is any solution, it has to be e^x. That is assuming that these derivatives follow the rules Dr. Peyam explained here : ua-cam.com/video/gaAhCTDc6oA/v-deo.html

  • @Rundas69420
    @Rundas69420 6 років тому +6

    The derivaive of a function is given by an integral?
    Feels like complex analysis though. :D

  • @spockfan2000
    @spockfan2000 6 років тому +1

    Awesome channel.
    The one thing I kind of don't like is that you stay in front of the stuff you're writing.
    Can you position the camera more like they do in BlackPenRedPen, so that you're never in front of what you're writing?
    Anyway, awesome channel. I loved the episode with the i-th derivative, by the way.

  • @bensnodgrass6548
    @bensnodgrass6548 6 років тому +25

    can you prove this alpha-derivative formula?? love your videos

    • @michalbotor
      @michalbotor 6 років тому +7

      first we would have to ask ourselves what do we mean by proving this formula in the first place, since it's a definition after all snickily defined only for 0 < a < 1. i suppose, that (D^a)o(D^(1-a)) = d/dx for all 0 < a < 1 is our only constraint. there are many other *distinct* definitions of alpha-derivatives out there btw, similarly to how the (euler's) gamma function is *not* the only valid extension of the factorial function, see hadamard's gamma function.
      however if you're asking for the intuition for it, then it is cleverly deduced from the cauchy formula for the repeated integration. just find definition of f^(-n)(x) on its wiki page, base it at a=0, substitute gamma(n) for (n-1)!, change n to alpha: 0 < alpha < 1, notice, that (x-t)^(alpha-1) = 1/(x-t)^alpha, and finally take the derivative d/dx of this formula and *define* it to be the alpha-derivative.

    • @xCorvus7x
      @xCorvus7x 6 років тому +2

      @@michalbotor
      Isn't 1/(x-t)^α equal to (x-t)^(-α), and not to (x-t)^(α-1)?

    • @epicmorphism2240
      @epicmorphism2240 4 роки тому

      With cauchy formula for repeated integration, than use fundemental thrm of calc and logic ceiling function

    • @epicmorphism2240
      @epicmorphism2240 4 роки тому

      Angel Mendez-Rivera it‘s not just the motivation, it‘s the derivitation of this firmula and at the same time also the proof.

    • @epicmorphism2240
      @epicmorphism2240 4 роки тому

      Angel Mendez-Rivera 1) you can prove the CFFRI by induction and yes i know it‘s only defined for positiv integers, but if you expand a function to more values, it‘s an analytic continuation. And the whole topic of fractional or complex derivatives is an AC. You can derive the CF with pos intgers and proof it, so you are can also expand it. 2) the gamma function isn‘t just a defined function for complex arguments. Yes you can‘t evaluate -1/2! with the definition of the factorial for postiv integers. But the factorial has some specific properties. It‘s recursiv, G(1)=1 and super convex. With these properties you can expand the factorial for complex numbers.

  • @k.c.sunshine1934
    @k.c.sunshine1934 5 років тому +1

    The D1/2 ln(x) has a zero in addition to it's expected pole! Very interesting.

  • @Absilicon
    @Absilicon 6 років тому +5

    Ok, you have a new a subscriber 😁

  • @benjaminbrat3922
    @benjaminbrat3922 6 років тому +4

    Not disappointed =)

  • @Tomaplen
    @Tomaplen 6 років тому +1

    Does D^(alpha) conmutes with D^n when 0

    • @drpeyam
      @drpeyam  6 років тому +2

      Yeah, that’s how you define D^alpha for alpha > 1 actually!

    • @Tomaplen
      @Tomaplen 6 років тому

      @@drpeyam Ooooh ok nice, thank you!!

  • @zathrasyes1287
    @zathrasyes1287 6 років тому +2

    Well Done. Could you make a video with an introduction into Fox H-funktion?

  • @christophem6373
    @christophem6373 6 років тому +2

    Is there application for half derivative, as derivative for variation of function, andtangete equation, and second derivative for convexity .... ?

    • @drpeyam
      @drpeyam  6 років тому +3

      Yeah, look at the pinned comment in my previous half derivative video

  • @ashwinvishwakarma2531
    @ashwinvishwakarma2531 6 років тому +1

    Is the reason we differentiate because if alpha>=0 then the integral diverges (I'm not sure if it does, since it doesnt diverges if 1>alpha>0)?

  • @scottgoodson8295
    @scottgoodson8295 6 років тому +12

    But now you have to take the half derivative of this to check that you get 1/x 😉

    • @asmodeojung
      @asmodeojung 5 років тому +3

      Nah, just take a half-integral of 1/x. Piece of cake... probably.

  • @cycklist
    @cycklist 6 років тому

    Absolutely superb. Love it!

  • @zwz.zdenek
    @zwz.zdenek 6 років тому +2

    What is the intuitive meaning of a non-whole-number derivative? Where is it useful?

    • @drpeyam
      @drpeyam  6 років тому +1

      See the pinned comment on the previous video

    • @zwz.zdenek
      @zwz.zdenek 6 років тому +1

      Thanks for replying, but I'm not at all familiar with fluid dynamics; I was hoping for getting the relationship between a graph of a function and its half derivative.
      I can see how these could be useful for fitting a model, but any model fitting is a virtue out of necessity and far from the exactness of mathematics.

    • @DrunkenUFOPilot
      @DrunkenUFOPilot 5 років тому

      I've been fascinated by this topic for many years. I still don't have a good intuitive understanding of semi-derivatives and semi-integrals, but I do know that integrating white noise makes brown noise. Half-integrating white noise makes pink noise, useful in audio testing. Also, the isochrone problem. Then there are uses in electrochemistry and fluid dynamics, and I think I once saw something concerning antenna design. But an elegant, "aha!" getting of the concept intuitively, not yet. A semi-integral is a mild low pass filter, in a way, but that just isn't good enough for a sense of "getting" it.

  • @flowergirlkaomoji7361
    @flowergirlkaomoji7361 6 років тому +2

    tried to solve this in the spare time I had during my calc 2B final; no wonder it was so hard! (I ran into a wall :/ ) very cool :)

  • @leonardromano1491
    @leonardromano1491 6 років тому +1

    These fractional derivatives are linear operators, so it should be natural to ask about the maximum domain, and eigenfunctions + eigenvalues.
    Since the eigenfunctions of whole derivatives are exponentials/ plane waves and they are just functions of fractional derivatives they should be simultaneously diagonalizable on a sufficient domain.
    The quest is: Are the eigenfunctions of the fractional derivative known on their maximum domain, and if they are, are they just plane waves?

    • @dalek1099
      @dalek1099 6 років тому +2

      Leonard Romano For a fractional derivative D(a/b) with eigenfunction f this implies that f is also an eigenfunction of D(a) by applying D(a/b) b times and the eigenfunctions of D(a) are just plane waves.
      However, this argument doesn't hold for irrational derivatives so it would be interesting if there are non plane wave eigenfunctions of irrational derivatives. I guess that would depend on whether eigenfunctions change in a limit of operators(approaching irrational using more and more accurate fractions)

    • @DrunkenUFOPilot
      @DrunkenUFOPilot 5 років тому

      D^a D^n f(x) = D^n D^a f(x), and D^n = D D^(n-1), and so on... you find that given some eigen function g(x) of D, such that D g(x) = c g(x), then D D^a g(x) = D^a D g(x) = c D^a g(x), so the eigenfunctions of D^a are the same as for just D. Of course, the differential equation D g(x) = c g(x) is one of the simplest, with the result g(x) = exp(cx).
      It all boils down to D^a exp(cx) = c^a exp(cx). Let a be imaginary, and knowing D^a is linear, we can perform D^a on arbitrary functions using Fourier transforms. I once did that to a Cassini image :)

  • @justinji1595
    @justinji1595 8 місяців тому

    Hi, does this work for alpha = a complex number? Such as i?

  • @szymon5830
    @szymon5830 6 років тому +4

    How about i-th direvative?

    • @szymon5830
      @szymon5830 6 років тому

      And how to do that? Is there any formula for that?

    • @drpeyam
      @drpeyam  6 років тому +1

      There’s a video on that!

  • @insouciantFox
    @insouciantFox 4 роки тому +1

    The way you write your math gives me flashbacks to ‘Nam - Calc I.
    My Calc I teacher was very persnickety about what form the answers could be in: only in “simplified, factored, reduced, elementary form”
    This meant a few things:
    1) No decimals of any kind for any reason. 1.4236 had to be 3559/2500.
    2) Any fraction must be extended to the entire expression, e.g., 1/2 x + x/ln(x) must be (ln(x)+2x)/(2lnx)
    3) Denominators and logs must be rationalized/ in elementary form: ln16/sqrt(136) must be ln(2)sqrt(34)/34
    4) If possible, all nat logs should be written as one function, e.g. .5ln(x)-ln(sinx) must be ln(x/(sinx)^2)/2
    5) Any polynomial that can be factored through any known algebraic method must be so
    6) No negative exponents. Ever.
    7) All logs must be natural or common. No log_2(x), only ln(x)/ln(2)
    8) Euler’s notation for derivatives (i.e. the “D” functional operator). Prime notation was only for f(x) and the like.
    9) Trig must be the most compact/ reduced form possible. No x/sinx, only xcscx etc. This may not seem annoying, but for really complex exp, it can get tedious.
    Liked him as a teacher and person. Scarred me as a mathematician.

    • @drpeyam
      @drpeyam  4 роки тому

      Good, I like your teacher haha

  • @radiotv624
    @radiotv624 6 років тому +1

    Can you construct a proof of the formula for D^α f(x) please

  • @damiandassen7763
    @damiandassen7763 6 років тому +1

    I have a question: do these half derivatives have any "real" meaning; i.e are they usefull in physics. I can't think of what they might mean. And I have a request: can you do the half derivative again to see if it truly works. Btw I don't doubt your expertise. :)

    • @drpeyam
      @drpeyam  6 років тому +1

      Yes, see the pinned comment on a previous video

    • @damiandassen7763
      @damiandassen7763 6 років тому +1

      Dr. Peyam's Show btw I forgot to mention. This was a great video. Keep up the good work.

  • @vaniragujana
    @vaniragujana 3 роки тому

    Sir,
    Is it possible to find general real derivative for ln(x).
    Eg: D^r(ln(x))
    Where r is real number

  • @yoavshati
    @yoavshati 6 років тому +3

    Can you use the alpha differentiation to solve integrals by doing D(-1)?

  • @pedrocusinato02
    @pedrocusinato02 6 років тому +2

    Calculate the half derivative of a constant

    • @drpeyam
      @drpeyam  6 років тому +3

      Coming on Monday ;)

  • @drdca8263
    @drdca8263 5 років тому

    Now I’m curious as to what the half integral of x^(-1/2) is.
    Edit: it appears to be x*sqrt(pi)/2 ? That is based on just plugging the formula into Wolfram alpha though, and may be mistaken.

    • @drpeyam
      @drpeyam  5 років тому +1

      That doesn’t seem right, are you sure you didn’t do the half integral of x^1/2 or the integral of x^-1/2?

    • @drdca8263
      @drdca8263 5 років тому

      Dr Peyam I did (1/sqrt(pi)) * integral from 0 to t of (t-x)^(1/2) * x^(-1/2) dt
      Maybe I should have used (t-x)^(-3/2) ? Using that, Wolfram alpha tells me that the integral does not converge. Is this what you would expect?

    • @drpeyam
      @drpeyam  5 років тому

      Ah, I know where the mistake is: The formula is only valid for alpha between 0 and 1, so to get the half integral, you take the integral and then the half derivative (using the formula above)

    • @pickleyt6432
      @pickleyt6432 Рік тому

      So I just calculated it and you should get zero as the result for D^(1/2)(1/sqrt(x)) via the formula D^n(x^k)=x^(k-n)Gamma(k+1)/Gamma(k+1-n), and plugging in k=-1/2. This is because the term in the bottom goes to Gamma(1/2-n), which approaches infinity as n approaches 1/2, thus making the whole expression tend towards zero

  • @txikitofandango
    @txikitofandango 4 роки тому

    So, if we let f(x) = ln(4x)/sqrt(pi*x), then f(f(x)) should just equal 1/x, right? Well, that's not the case when I graph it, so where am I wrong?

    • @txikitofandango
      @txikitofandango 4 роки тому

      In Desmos the graphs of f(f(x)) as defined above and 1/x look different.

  • @mattryan2006
    @mattryan2006 5 років тому

    if you switch variables to do it for ln(m) you get a factor of root Pi.m in the denominator

  • @GhostyOcean
    @GhostyOcean 6 років тому

    Why not use int(db/(a^2-b^2))=(1/a)arctanh(b/a)+c at 9:20, then use the definition of the arctanh function to find it in algebraic terms? You'll get the same answer with less work (assuming you didn't want to do that partial fraction)
    Edit: fixed some typos
    Edit 2: I guess you just didn't want to use it? To each their own 🤷‍♂️

    • @drpeyam
      @drpeyam  6 років тому

      Yeah, but I don’t like obscure integration formulas ;)

    • @GhostyOcean
      @GhostyOcean 6 років тому

      @@drpeyam ahhh I see. Seeing the integration in full is more useful to the casual watcher since they get to learn how to do it if they didn't already.

  • @lalitverma5818
    @lalitverma5818 6 років тому

    Very talented sir

  • @francescocapacci9941
    @francescocapacci9941 5 років тому

    I loved this calculus, but my question now is : does really exist a function which the half derivative is actually 1/x, like the log in the usual derivate??? I wonder what 'd be.

    • @drpeyam
      @drpeyam  5 років тому +1

      Yeah, that would be interesting! I wonder if 1/x^1/2 works!

  • @АлексейАкимов-щ8т
    @АлексейАкимов-щ8т 3 роки тому

    Что на 2:51 произошло с -1/2 ?

    • @АлексейАкимов-щ8т
      @АлексейАкимов-щ8т 3 роки тому

      И если что я дальше смотреть не стал. Потому-что мне уже то непонятно.

  • @digxx
    @digxx 4 роки тому

    What I always wonder, what the lower integration limit is supposed to be. You take x=0, but why not x=1? When integrating an integer number of times, this amounts to the successive integration constants coming in. When deriving an integer number of times they however vanish. But how many integration constants are there if you derive a half times = integrating a half times + deriving 1 time. When you choose x=1 as the lower bound since ln(t) vanishes there, you would instead get 2*\log(\sqrt{x}+\sqrt{x-1})/\sqrt{\pi x} which is somewhat different from \log(4x)/\sqrt{\pi x}. In fact, getting from the one to the other expression involves infinitely many integration constants c_n for the series \sum_{n=0}^\infty c_n x^n (this would only be finitely many terms when integrating an integer number of times).

  • @tricky778
    @tricky778 3 роки тому

    Rational derivatives are all very well, but what about irrational derivatives?

  • @thomasblackwell9507
    @thomasblackwell9507 5 років тому +1

    Who survived? In the words of the Great Bugs Bunny “I should have turned left at Albuquerque!”

  • @christophem6373
    @christophem6373 6 років тому +1

    In which theorical context this half derivative appear ?
    Same answer to my first question: Fractional Analysis.
    en.wikipedia.org/wiki/Fractional_calculus

  • @t.diyarmath605
    @t.diyarmath605 3 роки тому

    Hello Dr. this video it is so nice can you solve by Caputo fractional derivative

  • @MrRyanroberson1
    @MrRyanroberson1 6 років тому

    I think you could prove that this is the half derivative by taking the half derivative of the answer to get 1/x. You could also prove your prediction of 1/sqrt(x) wrong by half differentiating that as well

  • @adonaythegreat8426
    @adonaythegreat8426 2 роки тому

    What is the significance of half derivative

    • @drpeyam
      @drpeyam  2 роки тому

      See pinned comment on the original half derivative video

  • @thomaswilliams5320
    @thomaswilliams5320 6 років тому +1

    Can anyone explain why 2t*ln(t) = 0 when t = 0, it seems like this implies that 0^0 = 1

    • @jordancole4004
      @jordancole4004 6 років тому

      Thomas Williams wanna go park???

    • @BabyXGlitz
      @BabyXGlitz 3 роки тому

      2t*ln(t) = 2*ln (t^t)
      lim t^t as t goes to 0 is 1
      and 2ln(1) =0

  • @mokouf3
    @mokouf3 4 роки тому

    That integral is best for high school students, for scaring them!

  • @cachamuertos
    @cachamuertos 6 років тому +3

    why your profile photo is a rabbit? it is funyy XD

    • @drpeyam
      @drpeyam  6 років тому +3

      It’s my bunny Oreo!

    • @cachamuertos
      @cachamuertos 6 років тому +3

      Dr. Peyam's Show wow its cute.
      Ohh i just remembered something. Its possible to express cbrt(a+bi) in terms of p+qi?

    • @xy9439
      @xy9439 6 років тому

      Cesar Cruz Yes of course

  • @MiguelGonzalez-hy4sd
    @MiguelGonzalez-hy4sd 6 років тому +7

    The pi derivative?🌚🌚🌚

    • @drpeyam
      @drpeyam  6 років тому +3

      The 3rd derivative of the pi-3 derivative :)

  • @insouciantFox
    @insouciantFox 4 роки тому

    So it’s a derivative of a convolution. Interesting.

  • @paulbooer7171
    @paulbooer7171 6 років тому

    Wow! I need to lie down now and rest my throbbing brain!

  • @giovannisecondo7339
    @giovannisecondo7339 5 років тому

    Sorry if exist the half derivative so exist the half integration ?

  • @alberteinstein7571
    @alberteinstein7571 6 років тому +1

    👍 :D

  • @ReubenMason99
    @ReubenMason99 6 років тому

    Is D^1/2(D^1/2(f(x)))=D^1(f(x))

  • @jesusalej1
    @jesusalej1 4 роки тому

    Thank you for surviving? 😂😂😂

  • @tofu8676
    @tofu8676 5 років тому

    what a monstrosity to integrate

  • @warrickdawes7900
    @warrickdawes7900 6 років тому

    When you're trash-talking other maths channels :)

  • @محمدخليل-خ4ز9خ
    @محمدخليل-خ4ز9خ 6 років тому

    Its easy