I think few will oppose me if I say the best part of March 2020 was that it ended. Let me close out nearly all my March business by getting through the last couple comic strips which mentioned some mathematics topic that month. I’ll still have my readership review, probably to post Friday, and then that finishes my participation in the month at last.

Connie Sun’s Connie to the for the 30th features the title character trying to explain what “exponential growth” is. She struggles. Appropriately, as it’s something we see very rarely in ordinary life.

They turn up in mathematics all the time. And mathematical physics, and such. Any process with a rate of change that’s proportional to the current amount of the thing tends to be exponential. This whether growing or decaying. Even circular motion, periodic motion, can be understood as exponential growth with imaginary numbers. So anyone doing mathematics gets trained to see, and expect, exponentials. They have great analytic properties, too. You can use them to solve differential equations. And differential equations are so much of science that it’s easy to forget they’re not.

In ordinary life, though? Well, yes, a lot of quantities will change at rates which depend on their current quantity. But in anything that’s been around a while, the quantity will usually be at, or near enough, an equilibrium. Some kind of balance. It may move away from that balance, but usually, it’ll move back towards it. (I am skipping some complicating factors. Don’t worry about them.) A mathematician will see the hidden exponentials in this. But to anyone else? The thing may start growing, but then it peters out and slows to a stop. Or it might collapse, but that change also peters out. Maybe it’ll hit a new equilibrium; maybe it’ll go back to the old. We rarely see something changing without the sorts of limits that tamp the change back down.

Even the growth of infection rates for Covid-19 will not stay exponential forever, even if there were no public health measures responding to it. There can’t be more people infected than there are people in the world. At some point, the curve representing number of infected people versus time would stop growing more and more, and would level out, from a pattern called the logistic equation. But the early stages of this are almost indistinguishable from exponential growth.

Samson’s Dark Side of the Horse for the 29th is a comforting counting-sheep joke, with half-sized sheep counted as fractions of a whole sheep. Comforting little bit of business here.

Sam Hurts’s Eyebeam for the 30th describes one version of Zeno’s most famous paradox, and applies it to an event that already seems endless.

Todd Clark’s Lola for the 30th has a student asking what the end of mathematics is. And learning how after algebra comes geometry, trigonometry, calculus, topology, and more. All fair enough, though I’m surprised to see it put for that that of course someone who does enough mathematics will do topology. (I only have a casual brush with it myself, mostly in service to other topics.) But it’s nice to have it acknowledged that, if you want, you can go on learning new mathematics fields, practically without limit.

Ashleigh Brilliant’s Pot-Shots for the 30th just declares infinity to be a favorite number. Is it a number? … We have to be careful what exactly we mean by number. Allow that we are careful, though. It’s certainly at least number-adjacent.

John Zakour and Scott Roberts’s Maria’s Day for the 31st has Maria hoping to get out of new schoolwork. So she gets a review of fractions instead. Typical.

There were some more mathematically-themed comic strips last week. I’ll get to them in an essay at this link, sometime soon. Thanks for reading.

Reading the Comics, October 11, 2018: Under Weather Edition

I ended up not finding more comics on-topic on GoComics yesterday. So this past week’s mathematically-themed strips should fit into two posts well. I apologize for any loss of coherence in this essay, as I’m getting a bit of a cold. I’m looking forward to what this cold does for the A To Z essays coming Tuesday and Friday this week, too.

Stephen Beals’s Adult Children for the 7th uses Albert Einstein’s famous equation as shorthand for knowledge. I’m a little surprised it’s written out in words, rather than symbols. This might reflect that $E = mc^2$ is often understood just as this important series of sounds, rather than as an equation relating things to one another. Or it might just reflect the needs of the page composition. It could be too small a word balloon otherwise.

Julie Larson’s The Dinette Set for the 9th continues the thread of tip-calculation jokes around here. I have no explanation for this phenomenon. In this case, Burl is doing the calculation correctly. If the tip is supposed to be 15% of the bill, and the bill is reduced 10%, then the tip would be reduced 10%. If you already have the tip calculated, it might be quicker to figure out a tenth of that rather than work out 15% of the original bill. And, yes, the characters are being rather unpleasantly penny-pinching. That was just the comic strip’s sense of humor.

Todd Clark’s Lola for the 9th take the form of your traditional grumbling about story problems. It also shows off the motif of updating of the words in a story problem to be awkwardly un-hip. The problem seems to be starting in a confounding direction anyway. The first sentence isn’t out and it’s introducing the rate at which Frank is shedding social-media friends over time and the rate at which a train is travelling, some distancer per time. Having one quantity with dimensions friends-per-time and another with dimensions distance-per-time is begging for confusion. Or for some weird gibberish thing, like, determining something to be (say) ninety mile-friends. There’s trouble ahead.

Zach Weinersmith’s Saturday Morning Breakfast Cereal for the 10th proposes naming a particular kind of series. A series is the sum of a sequence of numbers. It doesn’t have to be a sequence with infinitely many numbers in it, but it usually is, if it’s to be an interesting series. Properly, a series gets defined by something like the symbols in the upper caption of the panel:

$\sum_{i = 1}^{\infty} a_i$

Here the ‘i’ is a “dummy variable”, of no particular interest and not even detectable once the calculation is done. It’s not that thing with the square roots of -1 in thise case. ‘i’ is specifically known as the ‘index’, since it indexes the terms in the sequence. Despite the logic of i-index, I prefer to use ‘j’, ‘k’, or ‘n’. This avoids confusion with that square-root-of-minus-1 meaning for i. The index starts at some value, the one to the right of the equals sign underneath the capital sigma; in this case, 1. The sequence evaluates whatever the formula described by $a_i$ is, for each whole number between that lowest ‘i’, in this case 1, and whatever the value above the sigma is. For the infinite series, that’s infinitely large. That is, work out $a_i$ for every counting number ‘i’. For the first sum in the caption, that highest number is 4, and you only need to evaluate four terms and add them together. There’s no rule given for $a_i$ in the caption; that just means that, in this case, we don’t yet have reason to care what the formula is.

This is the way to define a series if we’re being careful, and doing mathematics properly. But there are shorthands, and we fall back on them all the time. On the blackboard is one of them: $24 + 12 + 6 + 3 + \cdots$. The $\cdots$ at the end of a summation like this means “carry on this pattern for infinitely many terms”. If it appears in the middle of a summation, like $2 + 4 + 6 + 8 + \cdots + 20$ it means “carry on this pattern for the appropriate number of terms”. In that case, it would be $10 + 12 + 14 + 16 + 18$.

The flaw with this “carry on this pattern” is that, properly, there’s no such thing as “the” pattern. There are infinitely many ways to continue from whatever the start was, and they’re all equally valid. What lets this scheme work is cultural expectations. We expect the difference between one term and the next to follow some easy patterns. They increase or decrease by the same amount as we’ve seen before (an arithmetic progression, like 2 + 4 + 6 + 8, increasing by two each time). They increase or decrease by the same ratio as we’ve seen before (a geometric progression, like 24 + 12 + 6 + 3, cutting in half each time). Maybe the sign alternates, or changes by some straightforward rule. If it isn’t one of these, then we have to fall back on being explicit. In this case, it would be that $a_i = 24 \cdot \left(\frac{1}{2}\right)^{i - 1}$.

The capital-sigma as shorthand for “sum” traces to Leonhard Euler, because of course. I’m finding it hard, in my copy of Florian Cajori’s History of Mathematical Notations, to find just where the series notation as we use it got started. Also I’m not finding where ellipses got into mathematical notation either. It might reflect everybody realizing this was a pretty good way to represent “we’re not going to write out the whole thing here”.

Norm Feuti’s Retail for the 11th riffs on how many people, fundamentally, don’t know what percentages are. I think it reflects thinking of a percentage as some kind of unit. We get used to measurements of things, like, pounds or seconds or dollars or degrees or such that are fixed in value. But a percentage is relative. It’s a fraction of some original quantity. A difference of (say) two pounds in weight is the same amount of weight whatever the original was; why wouldn’t two percent of the weight behave similarly? … Gads, yes, I feel for the next retailer who gets these customers.

I think I’ve already used the story from when I worked in the bookstore about the customer concerned whether the ten-percent-off sticker applied before or after sales tax was calculated. So I’ll only share if people ask to hear it. (They won’t ask.)

When I’m not getting a bit ill, I put my Reading the Comics posts at this link. Essays which mention Adult Children are at this link. Essays with The Dinette Set discussions should be at this link. The essays inspired by Lola are at this link. There’s some mention of Saturday Morning Breakfast Cereal in essays at link, or pretty much every Reading the Comics post. And Retail gets discussed at this link.

Reading the Comics, May 8, 2018: Insecure http Edition

Last week had enough mathematically-themed comics for me to split the content. Usually I split the comics temporally, and this time I will too. What’s unusual is that somewhere along the week the URLs that GoComics pages provide switched from http to https. https is the less-openly-insecure version of the messaging protocol that sends web pages around. It’s good practice; we should be using https wherever possible. I don’t know why they switched that on, and why switch it on midweek. I suppose someone there knew what they were doing.

Tom Wilson’s Ziggy for the 6th of May uses mathematical breakthroughs as shorthand for inspiration. In two ways, too, one with a basically geometric figure and one with a bunch of equations. The geometric figure doesn’t seem to have any significance to me. The equations … that’s a bit harder. They’re probably nonsense. But it’s hard to look at ‘a’ and not see acceleration; the letter is often used for that. And it’s hard to look at ‘v’ and not see velocity. ‘x’ is often a position and ‘t’ is often a time. ‘xf – xi‘ looks meaningful too. It almost begs to be read as “position, final, minus position, initial”. “tf – ti” almost begs to be read as “time, final, minus time, initial”. And the difference in position divided by a difference in time suggests a velocity.

So here’s something peculiar inspired by looking at the units that have to follow. If ‘v’ is velocity, then it’s got units of distance over time. $\left(\frac{av}{V}\right)^2$ and $\left(\frac{av}{I}\right)^2$ would have units of distance-squared over time-squared. At least unless ‘a ‘or ‘V’ or ‘I’ are themselves measurements. But the square root of their sum then gets us back to distance over time. And then a distance-over-time divided by … well, distance-over-time suggests a pure number. Or something of whatever units ‘R’ carries with it.

So this equation seems arbitrary, and of course the expression doesn’t need to make sense for the joke. But it’s odd that the most-obvious choice of meanings for v and x and t means that the symbols work out so well. At least almost: an acceleration should have units of distance-over-time-squared, and this has units of (nothing). But I may have guessed wrong in thinking ‘a’ meant acceleration here. It might be a description of how something in one direction corresponds to something in another. And that would make sense as a pure number. I wonder whether Wilson got this expression from from anything, or if any readers recognize something that I should have seen right away.

Todd Clark’s Lola for the 7th jokes about being bad at mathematics. The number of days left to the end of school isn’t something that a kid should have trouble working out. However, do remember the first rule of calculating the span between two dates on the calendar: never calculate the span between two dates on the calendar. There is so much that goes wrong trying. All right, there’s a method. That method is let someone else do it.

Bud Fisher’s Mutt and Jeff for the 7th uses the form of those mathematics-magic games. You know, the ones where you ask someone to pick a number, then do some operations, and then tell you the result. From that you reverse-engineer the original number. They’re amusing enough tricks even if they are all basically the same. It’s instructive to figure out how they work. Replace your original number with symbols and follow the steps then. If you just need the number itself you can replace that with ‘x’. If you need the digits of the number then you’d replace it with something like “10*a + b”, to represent the numerals “ab”. Here, yeah, Mutt’s just being arbitrarily mean.

Paul Gilligan and Kory Merritt’s Poptropica for the 7th depicts calculating stuff as the way to act like a robot. Can’t deny; calculation is pretty much what we expect computers to do. It may hide. It may be done so abstractly it looks like we’re playing Mini Metro instead. This is a new comics tag. I’m sad to say this might be the last use of that tag. Poptropica is fun, but it doesn’t touch on mathematics much at all.

Gene Mora’s Graffiti for the 8th mentions arithmetic, albeit obliquely. It’s meant to be pasted on the doors of kindergarten teachers and who am I to spoil the fun?

Scott Hilburn’s The Argyle Sweater for the 9th is the anthropomorphic-numerals joke for this week. Converting between decimals and fractions has been done since decimals got worked out in the late 16th century. There’s advantages to either representation. To my eyes the biggest advantage of fractions is they avoid hypnotizing people with the illusion of precision. 0.25 reads as more exact than 1/4. We can imagine it being 0.2500000000000000 and think we know the quantity to any desired precision. 1/4 reads (to me, anyway) as being open to the possibility we’re rounding off from 0.998 out of 4.00023.

Another advantage fractions do have is flexibility. There are infinitely many ways to express the same number as a fraction. In decimals, there are at most two. If you’re trying to calculate something that would be more easily done with a denominator of 30 than of 5, you’re free to do that. Decimals can have advantages in computing, certainly, especially if you’re already set up to manipulate digits. And you can tell at a glance whether, say, 14/29th is greater or less than 154/317th. In case you ever find reason to wonder, I mean. I’m not saying either is always the right way to go.

Reading the Comics, November 25, 2017: Shapes and Probability Edition

This week was another average-grade week of mathematically-themed comic strips. I wonder if I should track them and see what spurious correlations between events and strips turn up. That seems like too much work and there’s better things I could do with my time, so it’s probably just a few weeks before I start doing that.

Ruben Bolling’s Super-Fun-Pax Comics for the 19th is an installment of A Voice From Another Dimension. It’s in that long line of mathematics jokes that are riffs on Flatland, and how we might try to imagine spaces other than ours. They’re taxing things. We can understand some of the rules of them perfectly well. Does that mean we can visualize them? Understand them? I’m not sure, and I don’t know a way to prove whether someone does or does not. This wasn’t one of the strips I was thinking of when I tossed “shapes” into the edition title, but you know what? It’s close enough to matching.

Olivia Walch’s Imogen Quest for the 20th — and I haven’t looked, but it feels to me like I’m always featuring Imogen Quest lately — riffs on the Monty Hall Problem. The problem is based on a game never actually played on Monty Hall’s Let’s Make A Deal, but very like ones they do. There’s many kinds of games there, but most of them amount to the contestant making a choice, and then being asked to second-guess the choice. In this case, pick a door and then second-guess whether to switch to another door. The Monty Hall Problem is a great one for Internet commenters to argue about while the rest of us do something productive. The trouble — well, one trouble — is that whether switching improves your chance to win the car is that whether it does depends on the rules of the game. It’s not stated, for example, whether the host must open a door showing a goat behind it. It’s not stated that the host certainly knows which doors have goats and so chooses one of those. It’s not certain the contestant even wants a car when, hey, goats. What assumptions you make about these issues affects the outcome.

If you take the assumptions that I would, given the problem — the host knows which door the car’s behind, and always offers the choice to switch, and the contestant would rather have a car, and such — then Walch’s analysis is spot on.

Jonathan Mahood’s Bleeker: The Rechargeable Dog for the 20th features a pretend virtual reality arithmetic game. The strip is of incredibly low mathematical value, but it’s one of those comics I like that I never hear anyone talking about, so, here.

Richard Thompson’s Cul de Sac rerun for the 20th talks about shapes. And the names for shapes. It does seem like mathematicians have a lot of names for slightly different quadrilaterals. In our defense, if you’re talking about these a lot, it helps to have more specific names than just “quadrilateral”. Rhomboids are those parallelograms which have all four sides the same length. A parallelogram has to have two pairs of equal-sized legs, but the two pairs’ sizes can be different. Not so a rhombus. Mathworld says a rhombus with a narrow angle that’s 45 degrees is sometimes called a lozenge, but I say they’re fibbing. They make even more preposterous claims on the “lozenge” page.

Todd Clark’s Lola for the 20th does the old “when do I need to know algebra” question and I admit getting grumpy like this when people ask. Do French teachers have to put up with this stuff?

Brian Fies’s Mom’s Cancer rerun for the 23rd is from one of the delicate moments in her story. Fies’s mother just learned the average survival rate for her cancer treatment is about five percent and, after months of things getting haltingly better, is shaken. But as with most real-world probability questions context matters. The five-percent chance is, as described, the chance someone who’d just been diagnosed in the state she’d been diagnosed in would survive. The information that she’s already survived months of radiation and chemical treatment and physical therapy means they’re now looking at a different question. What is the chance she will survive, given that she has survived this far with this care?

Mark Anderson’s Andertoons for the 24th is the Mark Anderson’s Andertoons for the week. It’s a protesting-student kind of joke. For the student’s question, I’m not sure how many sides a polygon has before we can stop memorizing them. I’d say probably eight. Maybe ten. Of the shapes whose names people actually care about, mm. Circle, triangle, a bunch of quadrilaterals, pentagons, hexagons, octagons, maybe decagon and dodecagon. No, I’ve never met anyone who cared about nonagons. I think we could drop heptagons without anyone noticing either. Among quadrilaterals, ugh, let’s see. Square, rectangle, rhombus, parallelogram, trapezoid (or trapezium), and I guess diamond although I’m not sure what that gets you that rhombus doesn’t already. Toss in circles, ellipses, and ovals, and I think that’s all the shapes whose names you use.

Stephan Pastis’s Pearls Before Swine for the 25th does the rounding-up joke that’s been going around this year. It’s got a new context, though.