I gave up on my dream of being a mathematician around 1970, but I still enjoy occasionally taking a gander at the field from afar, even if I can no longer follow the details. In recent years I’ve run into something called *p-*adic numbers that were so unintuitive I cracked my brains trying to understand them without result (the Wikipedia page, like all their math pages, was singularly unhelpful); now, via this MetaFilter post, I have come as close to real understanding as I am likely to thanks to Derek Muller’s Veritasium video (33 min.). I normally prefer to absorb information by reading, but even a well-written piece like this one by Kelsey Houston-Edwards only made sense to me after watching the video.

But this isn’t MathHat, and I’m bringing it here because of the odd term “*p-*adic.” The “*p*” stands for *prime*, but why “-adic”? It was apparently first used in James Pierpont’s *Lectures on the Theory of Functions of Real Variables* (1905), p. 92: “When *m* is used as base, the numbers *a* are said to be expressed in an *m-adic system*.” But he’s just said “When *m*=10, we have the decimal system”; why would you go from “decimal” to “-adic”? Anybody know the history of this terminology?

In older mathematical literature, the Greek-derived “dyadic” was not infrequently used instead of the Latin-derived “binary.” In fact, a rational number whose denominator is a power of 2 is still typically referred to as a “dyadic rational,” even if “binary” is now preferred in virtually all other contexts (e.g., “binary number system” in preference to “dyadic number system”). In any event, as a working mathematician, I have no doubt that “p-adic” is modelled on “dyadic.” And in case anyone’s wondering, “binary” and “ternary” do get generalised to “n-ary” in mathematical jargon.

Greek instead of Latin? Decadic -> m-adic.

Edit: Or what Simplicissimus said.

In any event, as a working mathematician, I have no doubt that “p-adic” is modelled on “dyadic.”Thanks very much, that makes perfect sense!

Perhaps this is of further interest.

Of course “tribadic” is not quite the same as “triadic”, although both are non-binary.

‘dyadic’, ‘n-adic’ and even ‘variadic’ are used commonly in programming, to talk of functions written prefix taking two, n or a variable number of arguments. OTOH operators written infix (like +, -, ×, ÷) are called ‘binary’; negation prefix – or factorial suffix ! called ‘unary’. There are ‘ternary’ operators.

If you’re wanting to talk in general about functions/operators that take two arguments, prefer ‘dyadic’; if you’re talking about numbers-of-arguments in general ‘adicity’.

(Not to be confused with ‘acidity’.)

Yes, OED has this for

-adic:But that –

adsuffix1is the same as we see in more general use:The etymology:

And so on, in perhaps over-

soignédetail.There is another noun meaning of

dyadic(or olderdyad) in mathematics, referring to a kind of notation that is particularly useful in mathematical physics dealing with vectors. TheOEDhas a definition, but it looks to be quite dated and/or compiled by lexicographers who did not really understand what made dyadics useful—meaning that it emphasizes the wrong aspects of the mathematical formulation.The main idea of a dyadic is expressed in the first sentence of the Wikipedia article about the topic: “In mathematics, specifically multilinear algebra, a dyadic or dyadic tensor is a second order tensor, written in a notation that fits in with vector algebra.” The point is to express a second-rank tensor as something that can be combined with other vectors and tensors via dot and cross products. The normal symbol for a dyadic has a double-headed arrow above it (where a vector would have just a right-pointing arrow), indicating that it can form separate and distinct vector products from the right and the left.

The secret of p-adic numbers is that if p is a prime number, \eg 3, then its powers, \eg 9 = 3^2, 27 = 3^3, 81 = 3^4 … are understood to decrease to zero as the exponent gets large. It’s like decimal numbers with infinitely many digits to the left of the period, and only finitely many (other than zero) to its right. [I’m not making this up.]

This leads to the idea that there are different (p-adic) number systems (fields), all containing the field of rational numbers \eg 7/19… The real number system is the only one of these that’s `connected’ in a certain (both formal and intuitive) sense, the (countably many) rest of them being `totally disconnected’.

To quote the arithmetic geometer Kazuya Kato: the real numbers, like the sun, fill our field of vision in the day; but at night the primes, like the stars, come out.

This video is also an extremely good introduction to p-adics:

https://www.youtube.com/watch?v=3gyHKCDq1YA

Thanks!

Yes, definitely from dyadic. Dyads and triads are also commonly used terms in network theory, including in the social sciences, for the same reason – a sociologist would talk about a dyad and not a pair or a binary relationship.

I was a mathematics major at university and was introduced to p-adic numbers as part of the very first course I took, but they were really unintuitive concepts to me for a long time. Rather than having a mental picture of what they were, I had to figure out their properties and solve problems involving them by using the definitions we were given. It probably took me a whole year to get at a basic level of understanding that the Veritasium video explains in about 30 minutes (this wasn’t the fault of the instructor at university was a fantastic communicator; there simply wasn’t time to devote a whole lecture to p-adics as there were so many concepts to introduce to us). I wish this video had been available back then.

Other terms:

polyadic‘accepting at least two arguments’,polyvariadic‘accepting at least one argument’,niladicornullary‘accepting exactly zero arguments’.I think you have to be careful in extending etymology to mathematical terminology. Beware of the whimsy. Because the terrain of mathematics constantly expands, neonymy is a permanent necessity. Once bad neonymy has taken root it is hard to change.

In good conscience I can’t leave this

It’s like decimal numbers with infinitely many digits to the left of the period, and only finitely many (other than zero) to its right…

as it stands, without emphasizing that

\vdash : [I am not making this up.] :

Whether you’re adding or multiplying, you carry on carrying to the left and sort of quit when you get tired, \cf also John H Conway, on Numbers and Games \qv.

This, in connection with the Chinese Remainder theorem, can be extremely useful in quite gritty applied computational problems but IANANTheorist. The Ramans tell me they use 3 for routine work and 691 for serious stuff.

What does \vdash mean?

It’s doubtless part of his LaTeX dialect.

It must actually be a custom macro (or from a more obscure package), since the usual thing is “\Vdash”—and LaTeX commands are case sensitive.

However, if you want to know what an unknown LaTeX command produces, it’s almost always findable by searching Google for “\[whatever] LaTeX” (not case sensitive).

Beware of the whimsy.Yes, it’s -adictive.

So it’s this? I didn’t know that symbol at all…

@ DM,

\vdash prints as something called a turnstile, currently used mostly to indicate adjoint functors, but it has an honorable etymology going back to the Principia Mathematica of Whitehead and Russell, where it means `it is asserted that’ or something similar; it’s usually prefixed to a proposition.

[? Recall the mythopoetic battle between Whitehead and Wittgenstein the poker-holder, over the definition of a logical proposition, resolved according to story when it was realized that for Whitehead a proposition was, well, a proposition, but for Wittgenstein it was something written on a little piece of paper.]

Sorry, I’m completely unfamiliar with all of this. Your comment doubled my knowledge of Whitehead.

I realized post-sending that my mythopoetic memory had conflated Whitehead with Russell and then after checking

https://en.wikipedia.org/wiki/Wittgenstein%27s_Poker

that never mind it was Popper if it was anyone at all.

Here’s a little more: Whitehead wrote a conundrous book called Process and Reality. All that many people understood in it, including yours truly, was:

#

The safest general characterization of the European philosophical tradition is that it consists of a series of footnotes to Plato.

#

That’s a fine example of thoughtful exaggeration.

I’ve seen arguments that all of rock and roll is the legacy of Chuck Berry…

it was Popper if it was anyone at all.The at least equally known confrontation between poker-wielding Wittgenstein and another ultra-intellectual Mathematician is with Alan Turing. The way I heard it they had a blazing row about something inconsequential, and never spoke again. (I’m failing to find any reference to support that memory.)

I hope that Oxbridge colleges (and indeed the LSE) have withdrawn all pokers.

something called a turnstile, … has an honorable etymology going back to the Principia Mathematica of Whitehead and Russell, where it means `it is asserted that’ or something similar; it’s usually prefixed to a proposition.Not it doesn’t mean ‘it is asserted that’. It means: it is derivable from given axioms using only rules of inference. Sometimes you see a formula written to the left of the turnstile, meaning the r.h. formula can be derived from the l.h. Or a comma-list of formulas to the left meaning derive from those taken together. Or a meta-symbol to the left denoting ‘all axioms and all formulae derivable from them so far’, plus this extra formula to give the derivation.

You can derive palpable untruths from untrue axioms. (Indeed that’s a useful test to make sure you’re not mis-deriving where the conclusion is true but the axioms turn out insufficient to derive it.) Then the turnstile doesn’t ‘assert’ anything more than the mechanical process of derivation.

And it’s the weakness of those forms of derivation that give rise to Gödel’s theorem.

The turnstile also gets used in programming languages meta-theory to mean this program ‘can’t go wrong’ — in a unhelpfully technical sense of ‘go wrong’.

As an indigenous LaTeX writer (it’s not a spoken language) I assumed the turnstile symbol was a capital T for True turned sideways. Humpty Dumpty and I use it rhetorically when needed to mean `I assert (something)’.

Your comment doubled my knowledge of Whitehead.Allow me to triple it: “A Unitarian is someone who believes in at most one God.”

Accurate.

p-адические always impressed me by inconveinence of the term.

The wanted a name for them and they wanted it to be an adjective, so they invented a suffix -adic.

But speakers don’t recognise it, and it is attached to… an ideogram or what is

p?Note, in Russian it is still Latin

p.In math contexts, “p” is often used to represent a prime (number) variable. “i” is used to represent an integer (number) variable

Curiously, ” ” is used to represent a blank (space) constant, not “b”. It’s all rather complex, where “i” does not represent an integer variable.

However, “␢”, named BLANK SYMBOL, can be used to represent a blank space in a visible way. It dates back to the keypunch era, when the semantics of “␢” and “␢␢” were often distinct. A more modern synonym is “␣”, named OPEN BOX.

Mathematicians don’t deal in variables; a symbol means what it means, and in the next paragraph it’s another symbol with the same name and a different meaning. (In a programming language, they would be constants in different scopes).

In this case, another

pdenoting a prime number that may or may not be the same as the previousp. (The conflation of symbols and the mathematical objects that they stand for is endemic, but usually not a problem).Yes, it is

pin Russian mathematical texts as well. What I meant is that the term intends to use the outer (syntactic) interface of the suffix -адический /-adic, but horrible things are happening at its inner/left/earlier (morphonological) interface/surface:((Something like “time-invariant” has something quite different from a

suffixas its right part)Not to mention that I thought your numbers were named for someone callled Radichevskii (or perhaps Paddy Chaevskii).

P-addic.

@PlasticPrime, rather the latter. In mathematical contexts I tend to read isolated letters that seem designate something as Latin: they seldom Cyrillic in formulas:)

But much of what I said relates to orthography. I think Russians just don’t say

peh ʔadi…as the spelling suggests. Unfortunately I keep reading it so each time I see it written:(But there is still no identifiable stem to which the suffix is attached… Unless it is

paddy-cheskij.Also in Russian “pee” is the name of the Greek letter pi.

п is “peh”, with a hard /p/.

Neither “

padi…” nor even “pe.adi…” sounds similar todiadaandtriada.I don’t know if this parallelism is maintained in English (/ˈdaɪ.æd/ but /piː/).

It is interesting that even in mathematical equations, there can turn out to be quite a bit of linguistic-like redundancy. In at least one of my published papers, I used the letter

etwo mean two different things, even within the same equations. One was the basis of the natural logarithm, and the other was the charge of the electron. However, there was no realistic possibility for confusion between the two, based on where they appeared in the equations involved. I don’t think anybody even noticed the ambiguity, except for the copyeditor who edited the LaTeX to match the journal style, who changed one of the types ofeto be printed upright, rather than in italic like most mathematical quantities. Of course, the editor did this entirely on their own, without needing to consult me, which just indicates that it was already transparent which type ofeeach instance was.On the other hand, I have seen multiple uses of

itrip people up once or twice. Most of the time, one is unlikely to confuse the square root of –1 with an integer-valued index, particularly when the indexionly appears in subscripts (or superscripts), like inbᵢ. However, sometimes the numerical value of the indexiactually comes into things (for example, in the usual metric used to show that a countable product of metrizable spaces is itself metrizable). That same journal also tried to print the imaginary unit in upright type, to distinguish the two meanings. However, I just try to avoid the confusion completely; I follow J. J. Sakurai and start my integer indices from the letterj. (Of course, electrical engineers usejfor the square root of –1, to avoid confusion with the currenti. This tends to get mocked by physicists, who figured out that we could just use a majuscule letter for the current instead).Dyadandtriadhave /aɪ/, but that has nothing to do with the parallelism. After alltetrad,pentad, etc, don’t have it.Parallelism exists when native speakers can (does not matter how) recognise dyad, triad…. based on the pronunciation of p-adic. In Russian I think we can’t, I can’t. But maybe the association works in the language (I don’t know which one) where the term was coined, or in English.