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- Hacker News
- IIRC, Knuth use lg for logarithm base 2.by psychoslave
- All this would be way more interesting if it actually helped to demonstrate a novel mathematical fact. Right now it's more like notational play.by kfse
- I happen to think that novel facts and theorems and proofs are way overrated. If you find a new fact it just goes into the giant pile of facts that are sitting around uselessly. The useful progress in math is comes from "refactoring" efforts to make things simpler and more intuitive.
I don't mean that this is necessarily the case, but that it is where we are now: we have found ourself in a situation where we have way too many facts and not enough simple perspectives that make them useful and accessible.
Just my opinion, though.
by ajkjk - I read this kind of essay as a certain part of the arc by which new thoughts are formed: an act of large-scale pattern matching, laying out a bunch of cases which resemble each other, searching for the essential basis of the resemblance.
To post such a pattern allows the thought process to become distributed. Perhaps someone else will see the insight.
by sixo - The term "baseless logarithm" is really nonsensical and using it would be a great mistake.
Nonetheless, where the author of TFA is correct is that logarithms are a single physical quantity, like length, area or volume, and that choosing the so called "base" is choosing the unit of measurement for logarithms.
Logarithms are included in the dimensional formulae of many derived physical quantities, e.g. for describing the attenuation or amplification of waves during their propagation, where one uses quantities like logarithm per length and logarithm per time.
Changing the "base" of logarithms modifies the numeric values of all derived physical quantities exactly in the same manner as changing any other fundamental unit of measurement, like the unit of length or the unit of time.
Like for any physical quantity, the complete value of a logarithm is independent of the unit of measurement, because it is the product between the numeric value and the unit of measurement. When the unit of measurement is changed, both the numeric value and the unit are changed and the product stays the same (i.e. the logarithm corresponds to the same ratio, regardless what base is used to compute a numeric value for the logarithm).
Nowadays, the unit of logarithms is normally chosen between the octave (binary logarithms), neper (hyperbolic logarithms) or bel (decimal logarithms).
The units of measurement for logarithms are not the bases, but the logarithms of the bases, which is why e.g. the value of the number "e", the base of the hyperbolic logarithms, is never needed in any computation. The only values that are needed are "ln 2" or its inverse "log2 e", which are used to convert the numeric values of logarithms when the unit of measurement is changed between those corresponding to binary logarithms and to hyperbolic logarithms (a.k.a. natural logarithms, but there is nothing more "natural" about hyperbolic logarithms than about any other kind of logarithms).
by adrian_b - "Baseless logarithm" is not nonsencial. Given that:
a baseless logarithm is simply a family of functions with similar properties. Perhaps it might be clearer if the author said something like the "logarithm property" rather than "baseless logarithm" but that's nit-picking and debatable.d(logₐx)/dx = 1/(x log(a))As for changing the base changes the numbers, I have to wonder if you've done any advanced linear algebra or, more specifically, tensors. The whole point of a tensor is that it operates the same on an object regardless of the basis. Put another way, if a and b are two representations of the same object with different bases then T(a) and T(b) are equivalent if T(x) is a tensor.
My point is that any numbers are an arbitrary choice and they don't define the underlying structure. The author here is talking about logarithmic structure.
This btw is why you learn about different bases in linear algebra and converting between them. Or even polar coordinates vs cartesian coordinates (in high school, for some reason). They're priming you to learn about structure. You get to groups and learn that group A and B are isomorphic they have the same mathetmatical structure.
Even when the numbers change.
by jmyeet - I can't believe he called normal logarithms 'based'
- The same idea comes up in physics. In quantum physics, the action S appears as the logarithm-like quantity behind the amplitude e^iS/(h^bar). In statistical mechanics, entropy is the logarithm of the number of possible microstates Omega : S = log(Omega). Although the concepts come from different parts of physics, they both reflect the same principle: using a log as a way to turn multiplicative relationships into additive ones.by GL26
- >You might ask: if we have a baseless logarithm log(N), do we also have a “baseless exponential”?
Sure we can, with some naive algebra. If we can take log(x,base) and drop the base, then we can also take pow(base,x) and drop the base. Since bits=log(2), then pow(bits)=2. You can probably connect it to the reverse of things, like integrals.
Also, for fun, I'll play with some notation tricks.
I like the intuition that baseless logarithm notation gives, and it also avoids needing to choose a specific reference point. I can also directly calculate by choosing an arbitrary base:log(freq) = pitch freq = pow(pitch) octave = log(2) 400*Hz = 100*Hz*4 // the frequency 400 Hz equals 4 times 100 Hz log(400*Hz) = log(100*Hz) + log(4) log(400*Hz) = log(100*Hz) + 2*log(2) log(400*Hz) = log(100*Hz) + 2*octave log(400*Hz) = log(100*Hz) + 2*octave // the pitch of 400 Hz equals 2 octaves above the pitch of 100 Hz cent = log(2)/1200 A4 = log(440*Hz) B4 = A4 + 200*cent // the pitch B4 equals 200 cents above A4 B4 = log(440*Hz) + 200*log(2)/1200 B4 = log(440*Hz) + log(2^(2/12)) B4 = log(440*Hz * 2^(2/12)) pow(B4) = 493.883 Hz // the frequency of B4 equals 493.883 Hzpow(log(440*Hz) + 200*log(2)/1200) exp(ln(440) + 200*ln(2)/1200)by amavect - True, I guess you can just 'curry' exponentiation and say that's a baseless power. I couldn't find a clean notation for it so I gave up..by ajkjk
- Hah, I can use this to give decibels an actual unit.
Very helpful!! Imagine combining the goofy list of decibel suffixes into a uniform notation. Write the logarithm first so the + or - stays in the same spot.dB_P = log(10)/10 dB_F = log(10)/20 log(10*V) = log(V) + 20*dB_F // the level of 10 V equals 20 dB more than the power level of 1 V. SPL = 20*10^-6 * Pa hearing_damage = log(SPL) + 90*dB_F // hearing damage occurs over 90 dB_F above SPL (neglecting A-weighting) pow(hearing_damage) = pow(log(SPL) + 90*dB_F)) pow(hearing_damage) = pow(log(SPL) + 90*log(10)/20)) pow(hearing_damage) = SPL*pow(90*log(10)/20)) pow(hearing_damage) = SPL*31622.7766 // the pressure of hearing damage occurs above 31622 times SPL pow(hearing_damage) = 0.632455532 Pa // the pressure of hearing damage occurs above 0.632 Pa
https://en.wikipedia.org/wiki/Decibel#List_of_suffixeslog(reference_unit) + value*dB_F (or dB_P) log(reference_unit) - value*dB_F (or dB_P)by amavect - Charles Petzold's The Lost Art of Logarithms is a great read (still a work in progress).by orc00
- This looks great; thanks for the pointer.
Charles Petzold's writings are always very clear and in-depth.
by rramadass - I think what's going on with the complex logarithm is basically the same as the logarithm that outputs the set of all possible bases for a vector space. The complex logarithm produces a Z-torsor, and the basis logarithm produces a GL(V)-torsor. There's probably some way to represent a choice of branch cut as a part of the choice of the base of the complex logarithm, and similarly the choice of a specific basis as part of the choice of base of the vector space base logarithm.by aesthesia
- Interesting, it did not occur to me of those as two instances of the same phenomenon. Although I still find the complex analytic one hard to think about.by ajkjk
- This essay needs a type system. Every time it says “log” it should say: log of what, into what?
It’s like audio where people say "dB" as if it answers the next question. Relative to what, measured how, and weighted for whom?
Author should brush up on https://en.wikipedia.org/wiki/Lie_theory
by badlibrarian - I still don't understand why audio dB are negative. That's relative to what? What happens at 0dB?by jfengel
- The first section details how the author thinks of "log N" with no base as an abstract object rather than a number. Or what are you referring to?by jmyeet
- The important properties of the logarithm are structural: we usually do not care about units or bases, except when carrying out an actual numerical computation.
As developed in the article, informally, but somewhat sufficiently, the change of base formula shows that the choice of base is largely irrelevant: different bases give equivalent logarithms up to a constant factor.
The Taylor expansion of exp gives a more intrinsic and general definition of the exponential function. This allows exp to be generalised structurally to many algebraic settings, provided the relevant convergence conditions are met: for example, the complex exponential and its many possible logs, the matrix exponential, and so on…
by rq1 - That's a lot of ways to think about logarithms.
Logarithms are laughably simple once you've fully internalized the meaning of the log function; it simply answers the question:
"To what power must I raise the base to get the argument?"
This is why the output tapers out as you increase the argument; because even if you increase the argument exponentially, you only need a fixed increment in the power to reach that number... So if you increase the argument only by a fixed amount (linearly) instead of exponentially, then it makes sense that the output will grow sub-linearly.
I remember when I was doing algebra with logs many years ago at school, I was applying rules to remove the log from one side of the equation.
Then when I got to uni, I had to revise the rules but it was kind of silly of me because those rules can be trivially derived if you just think about what the log function means. Turns out I had been solving equations with logs throughout school without understanding what they even meant... It's only at university that I actually bothered to learn them.
Actually TBH. I didn't even fully understand powers for some time even though I was doing calculus with them at school. I only fully understood powers once I properly internalized the concept of k-ary trees as a proxy.
It's one thing to be able to apply something, another to understand it. And I think to innovate with something, as a tool, it's not enough to be able to apply it. You must understand it.
by jongjong - What made you want to understand it or did it happen upon you in college
- A better way to understand logarithms is to start with the original motivation from Napier himself (https://sites.pitt.edu/~super1/lecture/lec44911/005.htm);
Seeing there is nothing (right well-beloved Students of the Mathematics) that is so troublesome to mathematical practice, nor that doth more molest and hinder calculators, than the multiplications, divisions, square and cubical extractions of great numbers, which besides the tedious expense of time are for the most part subject to many slippery errors, I began therefore to consider in my mind by what certain and ready art I might remove those hindrances. And having thought upon many things to this purpose, I found at length some excellent brief rules to be treated of (perhaps) hereafter. But amongst all, none more profitable than this which together with the hard and tedious multiplications, divisions, and extractions of roots, doth also cast away from the work itself even the very numbers themselves that are to be multiplied, divided and resolved into roots, and putteth other numbers in their place which perform as much as they can do, only by addition and subtraction, division by two or division by three.
This is what provides the intuition viz; convert multiplication/division/etc. of large numbers into addition/subtraction of two other smaller numbers. Logarithms as inverse of Exponentiation came much later. Starting with this generally confuses the student since they do not understand the point of it all.
From https://en.wikipedia.org/wiki/History_of_logarithms;
Napier conceived the logarithm as the relationship between two particles moving along a line, one at constant speed and the other at a speed proportional to its distance from a fixed endpoint.
Since the speed is directly proportional to its remaining distance from the fixed endpoint, it therefore is a deceleration, which results in the characteristic "flattening" of the curve.
Further details for understanding the above can be found at Priority, Parallel Discovery, and Pre-eminence: Napier, Burgi and the Early History of the Logarithm Relation (pdf) - http://www.numdam.org/item/RHM_2012__18_2_223_0.pdf
by rramadass - Logs are awesome. I started a math textbook from the 1920's a while ago, and all the calculations relied on tabulated logs, where you would convert the number to a log in a table to reduce the operation's degree, then convert back to the ordinary representation. This would reduce operations like finding cubed roots to division, would could be converted to log-log to be further reduced to subtraction before you would restore to ordinary notation. It feels like you're using a magic wormhole or something when you're doing this stuff by hand, it's really neat.
- care to share the name of the said book?
- Got a PDF? I love old books like this.by all2
- Yep, we used manual math + some log tables for calculations in our school exams as late as last decade. Since calculators were not allowed. The exam would be such that you would need the log tables once or twice over the course of the exam. Example: dividing = lookup(a)-lookup(b) and then lookup that in the inverse log (i.e exp) tables.
- The physical version of that magic wormhole is called a slide rule.by badlibrarian
- The baseless log here is just a torsor [0]!
Lots of things are torsors: position, currency values, calendar dates etc. the vales themselves are arbitrary, and translating/scaling them by some value doesn't make a functional difference. Torsors let us talk about these things without needing to make such an arbitrary choice a priori.
In the case of baseless logs, the underlying set is "information units", i.e. log 2 is bits, log e is nats, log 10 is digits, etc. The conversion factors give us the torsor's group, and picking a privileged unit is just a trivialization of the torsor.
The vector division notation is, similarly, encoding a g-torsor in precisely the same way as length units are.
The examples so far are all torsors with abelian groups, but specifying position both requires choosing an origin and a length unit. The group of this torsor is a suitable semidirect product between translation and scaling, which gives a non-abelian group.
Most of the time we just implicitly choose a trivialization, which often causes confusion because it identifies objects with operations on them, e.g. conflating vectors as positions with vectors as translations. The author's treatise on problems with geometric algebra [1] even brings up this point!
[0]:https://math.ucr.edu/home/baez/torsors.html
[1]:https://alexkritchevsky.com/2024/02/28/geometric-algebra.htm...
by xelxebar - Thanks for sharing, very interesting. I wonder how this maps to swe
- I do know about torsors actually but I didn't think to link it from there. I guess I don't find the term very useful; it feels like things are still hard to think about even after you know it's a torsor!---but also, I think I need to get more familiar with the concept, because the other commenter on here who described my basis-logarithm as a "GL(V)-torsor" really said it much more succinctly than what I was hacking out manually.
Regardless of the terminology, I thought it was interesting because I have never seen the logarithm thought about in that way.
by ajkjk