## The Set Tour, Part 3: R^n

After talking about the real numbers last time, I had two obvious sets to use as follow up. Of course I’d overthink the choice of which to make my next common domain-and-range set.

## R^{n}

**R ^{n}** is pronounced “are enn”, just as you might do if you didn’t know enough mathematics to think the superscript meant something important. It does mean something important; it’s just that there’s not a graceful way to say what offhand. This is the set of n-tuples of real numbers. That is, anything you pick out of

**R**is an ordered set of things all of which are themselves real numbers. The “n” here is the name for some whole number whose value isn’t going to change during the length of this problem.

^{n} So when we speak of **R ^{n}** we are really speaking of a family of sets, all of them similar in some important ways. The things in

**R**look like pairs of real numbers: (3, 4), or (4π, -2e), or (2038, 0.010010001), pairs like that. The things in

^{2}**R**are triplets of real numbers: (3, 4, 5), or (4π, -2e, 1 + 1/π). The things in

^{3}**R**are quartets of real numbers: (3, 4, 5, 12) or (4π, -2e, 1 + 1/π, -6) or so. The things in

^{4}**R**are probably clear enough to not need listing.

^{10} It’s possible to add together two things in **R ^{n}**. At least if they come from the same

**R**; you can’t add a pair of numbers to a quartet of numbers, not if you’re being honest. The addition rule is just what you’d come up with if you didn’t know enough mathematics to be devious, though: add the first number of the first thing to the first number of the second thing, and that’s the first number of the sum. Add the second number of the first thing to the second number of the second thing, and that’s the second number of the sum. Add the third number of the first thing to the third number of the second thing, and that’s the third number of the sum. Keep on like this until you run out of numbers in each thing. It’s possible you already have.

^{n} You can’t multiply together two things in **R ^{n}**, though, unless your n is 1. (There may be some conceptual difference between

**R**and plain old

^{1}**R**. But I don’t recall seeing a mathematician being interested in the difference except when she’s studying the philosophy of mathematics.) The obvious multiplication scheme — multiply matching numbers, like you do with addition — produces something that doesn’t work enough like multiplication to be interesting. It’s possible for some n’s to work out schemes that act like multiplication enough to be interesting, but for the most part we don’t need them.

What we will do, though, is multiply something in **R ^{n}** by a single real number. That real number is called a “scalar”. You do the multiplication, again, like you’d do if you were too new to mathematics to be clever. Multiply the first number in your thing by the scalar, and that’s the first number in your product. Multiply the second number in your thing by the scalar, and that’s the second number in your product. Multiply the third number in your thing by the scalar, and that’s the third number in your product. Carry on like this until you run out of numbers, and then stop. Usually good advice.

That you can add together two things from **R ^{n}**, and you can multiply anything in

**R**by a scalar, makes this a “vector space”. (There are some more requirements, but they amount to addition and multiplication working like you’d expect.) The term means about what you think; a “space” is a … well … something that acts mathematically like ordinary everyday space works. A “vector space” is a space where the things inside it are vectors. Vectors are a combination of a direction and a distance in that direction. They’re very well-represented as n-tuples. They get represented as n-tuples so often it’s easy to forget that’s just a convenient way to write them down.

^{n} This vector space property of **R ^{n}** makes it a really useful set.

**R**corresponds naturally to “the points on a flat surface”.

^{2}**R**corresponds naturally to an idea of “all the points in normal everyday space where something could be”. Or, if you like, it can represent “the speed and direction something is travelling in”. Or the direction and amount of its acceleration, for that matter.

^{3} Because of these mathematicians will often call **R ^{n}** the “n-dimensional Euclidean space”. The n is about how many components there are in an element of the set. The “space” tells us it’s a space. “Euclidean” tells us that it looks and works like, well, Euclidean geometry. We can talk about the distance between points and use the ideas we had from plane or solid geometry. We can talk about angles and areas and volumes similarly. We can do this so much we might say “n-dimensional space” as if there weren’t anything but Euclidean spaces out there.

And this is useful for more than describing where something happens to be. A great number of physics problems find it convenient to study the position and the velocity of a number of particles which interact. If we have N particles, then, and we’re in a three-dimensional space, and we’re keeping track of positions and velocities for each of them, then we can describe where everything is and how everything is moving as one element in the space **R ^{6N}**. We can describe movement in time as a function that has a domain of

**R**and a range of

^{6N}**R**, and see the progression of time as tracing out a path in that space.

^{6N}We can’t draw that, obviously, and I’d look skeptically at people who say they can visualize it. What we usually draw is a little enclosed space that’s either a rectangle or a blob, and draw out lines — “trajectories” — inside that. The different spots along the trajectory correspond to all the positions and velocities of all the particles in the system at different times.

Though that’s a fantastic use, it’s not the only one. It’s not required, for example, that a function have the same **R ^{n}** as both domain and range. It can have different sets. If we want to be clear that the domain and range can be of different sizes, it’s common to call one

**R**and the other

^{n}**R**if we aren’t interested in pinning down just which spaces they are.

^{m} But, for example, a perfectly legitimate function would have a domain of **R ^{3}** and a range of

**R**, the reals. There’s even an obvious, common one: return the size, the magnitude, of whatever the vector in the domain is. Or we might take as domain

^{1}**R**, and the range

^{4}**R**, following the rule “match an element in the domain to an element in the range that has the same first and third components”. That kind of function is called a “projection”, as it gives what might look like the shadow of the original thing in a smaller space.

^{2} If we wanted to go the other way, from **R ^{2}** to

**R**as an example, we could. Here set the rule “match an element in the domain to an element in the range which has the same first and second components, and has ‘3’ and ‘4’ as the third and fourth components”. That’s an “embedding”, giving us the idea that we can put a Euclidean space with fewer dimensions into a space with more. The idea comes naturally to anyone who’s seen a cartoon where a character leaps off the screen and interacts with the real world.

^{4}
## howardat58 4:34 pm

onWednesday, 7 October, 2015 Permalink |If you want to take this further, but only on the way to Fourier series, have a look at this piece of mine on non-geometrical vectors:

http://mathcomesalive.com/page4.html#nongeo

Maybe you can tidy it up a bit, or I can post it, or you could do a guest post.

I used this approach with my students before embarking on the Fourier stuff.

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## Joseph Nebus 7:04 pm

onWednesday, 7 October, 2015 Permalink |Oh, I like that representation, thank you. I’d be glad to use it if I may.

While I’ve certainly used the idea of the Fourier series as a point in space I haven’t gone out representing that, not for specific functions, and that might make a difference in how new students understand it.

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## howardat58 7:40 pm

onWednesday, 7 October, 2015 Permalink |Go for it !

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## Joseph Nebus 2:39 pm

onFriday, 9 October, 2015 Permalink |Thanks! Shall see what I can schedule in.

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## Reading the Comics, October 5, 2015: Boxes and Hyperboxes Edition | nebusresearch 2:00 pm

onFriday, 9 October, 2015 Permalink |[…] The Set Tour, Part 3: R^n […]

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## The Set Tour, Part 4: Complex Numbers | nebusresearch 2:04 pm

onThursday, 15 October, 2015 Permalink |[…] The Set Tour, Part 3: R^n […]

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## The Set Tour, Part 5: C^n | nebusresearch 12:04 pm

onFriday, 23 October, 2015 Permalink |[…] reference to Rn, another friend, probably tipped you off to the rest. The items in Cn are n-tuples, ordered sets of some number n of numbers. Each of those numbers is […]

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## The Set Tour, Part 7: Matrices | nebusresearch 3:01 pm

onFriday, 6 November, 2015 Permalink |[…] when I said “x” was the multiplication symbol. R2 x 3 is not a longer way of saying R6, an ordered collection of six real-valued numbers. The x does represent a kind of product, though. What we mean by R2 x 3 is an ordered collection, […]

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## The Set Tour, Part 8: Balls, Only Made Harder | nebusresearch 3:00 pm

onMonday, 16 November, 2015 Permalink |[…] course, I wrote down Sn rather than just S2. As with Rn and with Rm x n, there is really a family of similar domains. They are common enough to share a […]

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## The Set Tour, Part 10: Lots of Spheres | nebusresearch 3:00 pm

onThursday, 10 December, 2015 Permalink |[…] of Bn. But it’d be too much work to change things now. The third has lurked implicitly since all the way back to Rn, a set of n real numbers for which the ordering of the numbers matters. (That is, that the set of […]

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