On April 11, 2020, John Horton Conway died of COVID-19 on the age of 82 in New Brunswick, N.J. The areas of analysis coated by this exceptional mathematician included group concept, node concept, geometry, evaluation, combinatorial sport concept, algebra, algorithmics and even theoretical physics.

Conway’s inclinations and expertise led him to invent a exceptional mobile automaton referred to as the Sport of Life, which continues to fascinate after 50 years. Conway additionally devised elegant puzzles for packing packing containers of blocks that may solely be solved effectively with intelligent reasoning.

In keeping with Conway, his most vital contribution was his conceptualization of a__ __marvelous system of numbers referred to as surreal numbers. This class includes integers, actual numbers, transfinite numbers and infinitesimals—a construction that nobody beforehand imagined was potential through which all the pieces could be added, multiplied, and so forth.

Individuals who labored with Conway report that he thought so quick that no sooner did he hear an issue said than he usually already had an answer. Conway’s dedication to arithmetic for everybody led him to work on puzzles that delight followers of leisure arithmetic, such because the well-known Collatz conjecture (which is mentioned towards the tip of this text).

Conway was the mathematician most cited (extra then 30 occasions!) in my columns in *Pour la Science*, the French version of *Scientific American*. In making ready the articles, I might usually come throughout a consequence he had demonstrated or an vital concept that he had been the primary to suggest on the subject, a lot to my shock. Conway appreciated easy, sensible mathematical issues that referred to as on his creativity.

The easiest way to honor him, it appears to me, is to spotlight the type of arithmetic that amazed and fascinated him. I’m going to cowl a number of completely different matters to which Conway contributed. However it could take a number of volumes to do justice to this distinctive thoughts. He was in a position to invent objects and issues in many various domains and to resolve probably the most recalcitrant puzzles, conceiving strategies nobody may have imagined.

## The Irrationality of √2

Probably the most shocking and vital mathematical findings is the irrationality of the sq. root of two (√2)—the size of the diagonal of a sq. with sides which can be one unit lengthy.** ** It can’t be expressed by the quotient of constructive integers *n *and *m, *or *n/m*. The invention of the irrationality of √2 is credited to Pythagoras or one in all his disciples, though we have no idea whether or not the reasoning behind it was arithmetic or geometric. The invention and its proof had been profoundly unsettling for mathematicians. This primary damaging discovering in arithmetic confirmed that people don’t create the legal guidelines governing numbers however somewhat uncover them as they discover uncharted mathematical territory.

Although there are various proofs of this theorem, probably the most intuitive is a quite simple little drawing that Conway included in a lecture printed in a ebook in 2005. ** **He attributed the creation of the proof to mathematician Stanley Tennenbaum, who, in accordance with Conway, had deserted arithmetic. You would possibly ask whether or not it was Conway himself who formulated the proof. However that doesn’t matter. As a result of even when he didn’t create it, the proof gives an ideal instance of Conway’s strategy to arithmetic, which he demonstrated in 100 other ways. It additionally exhibits that it’s flawed to imagine that all the pieces easy has already been found: good but astonishingly easy concepts are nonetheless ready to be revealed.

Say that √2 is the quotient of *n *and *m*—that’s, 2 = *n*^{2}/*m*^{2}*,* or 2*m*^{2} = *n*^{2}. In that case, there exists a sq., with sides equal to *n,* whose space equals twice that of a sq. with sides equal to *m* [*see part A of “An Irrational Square Root”*]. We assume that in our drawing, *m* is the smallest constructive integer satisfying this equation. The belief can be legitimate provided that there is no such thing as a smaller constructive integer that satisfies it.

Wedging the 2 blue squares into two diagonally reverse corners of the pink sq. produces a brand new form. The 2 pink squares in that form should have the identical space because the central purple sq. that’s created the place the 2 blue squares overlap [*see Part B of “An Irrational Square Root”*].

Our reasoning requires that the realm of the 2 blue squares should be equal to the realm of the pink one (*A*). Consequently, the realm not coated by the 2 blue squares is the same as twice the realm coated by each of them. In different phrases, there are two equal and smaller squares (*pink, B*) that collectively have the identical space because the bigger sq. (*purple, B*). Which means both sides of the brand new small squares is the same as the integer *n *–* m*, and both sides of the massive purple sq. is the same as the integer *n* – 2(*n* – *m*) = 2*m* – *n*.

Briefly, the preliminary sq. of facet *m* was not the smallest potential one which satisfies the geometric equation. The result’s a contradiction, and thus the belief is fake: √2 just isn’t a quotient of two integers, which suggests it’s an irrational quantity. In the identical approach, you’ll be able to display that the sq. root of three is irrational [*see part C of “An Irrational Square Root”*].

## Three Dice-Packing Puzzles

Many puzzles include placing a small variety of blocks in a field (say 10). Each the blocks and the field are sometimes rectangular parallelepipeds. The answer is normally arrived at by trial and error. With an issue like this one, except you improve the variety of blocks and their number of shapes, it’s exhausting to conceive a difficult puzzle. Furthermore, though it may be enjoyable to govern the blocks till the answer is discovered, doing so includes solely slightly reasoning about form and thus does probably not scale back the answer time.

Conway reimagined the issue by creating puzzles that may probably lead you in circles whilst you strive to determine find out how to fill the field. However just a few astute and orderly issues will rapidly information you to the answer.

The best of those three puzzles consists of filling a 3 x 3 x 3 field with three 1 x 1 x 1 cubes and 6 1 x 2 x 2 parallelepipeds [*see part 1 of* *“Blocks in a Box”*].

The reasoning consists of taking parity under consideration: The three x 3 x 3 field consists of three horizontal 3 x 3 x 1 layers, every with a quantity of 9. The three x 3 x 3 field may also be decomposed into three vertical 3 x 3 x 1 parallelepipeds, every parallel to a vertical face of the field. Contemplating the perpendicular vertical face produces a 3rd decomposition of the field into three 3 x 3 x 1 parallelepipeds.

Let’s study these 9 parallelepipeds. When a 1 x 2 x 2 block is positioned within the field, it might probably solely fill an excellent variety of the 9 cells in every 3 x 3 x 1 layer. So every layer should comprise one—and just one—of the three small 1 x 1 x 1 blocks (in any other case you would need to place all three of them in a single 3 x 3 x 1 layer, which would go away none within the different two layers, the place they’re wanted). The higher layer of the field should subsequently comprise one 1 x 1 x 1 block. If we place it within the heart of the layer, we rapidly attain a lifeless finish, as a result of we’re obliged to put one other small block slightly below it. That places two of them in a single 3 x 3 x 1 vertical layer. Putting the small block on the highest face in the course of any facet additionally leads to an deadlock. Consequently, the small block on the highest layer should go in a nook.

The identical reasoning applies to the underside layer, the place a small block should be positioned in a nook. That nook should be diametrically reverse to the small block on the highest layer. The third small block should subsequently go within the heart of the center layer. The remaining steps turn out to be rapidly apparent. Word that this reasoning not solely produces an answer but additionally exhibits that, apart from symmetries, there is just one resolution.

The answer to a different of Conway’s packing issues is proven partially 2 of “Blocks in a Field.”** **To finish the packing, all it’s important to do is to decrease the 2 higher buildings and place the inexperienced layer (*b*) on the facet on prime of the pink layer (*a*). Reasoning based mostly on parity forces us to put the unit cubes alongside the diagonal.

## The Riddle of the Two Wizards

Within the Sixties Conway devised a fiendishly difficult puzzle that, till just lately, prompted a lot debate and the publication of a paper by Tanya Khovanova of the Massachusetts Institute of Expertise in 2013. Right here is the puzzle because it seems in that paper:

Final evening I sat behind two wizards on a bus, and overheard the next:

A: “I’ve a constructive integral variety of youngsters, whose ages are constructive integers, the sum of which is the variety of this bus, whereas the product is my very own age.”

B: “How attention-grabbing! Maybe in case you instructed me your age and the variety of your youngsters, I may work out their particular person ages?”

A: “No.”

B: “Aha! AT LAST I understand how previous you might be!”

Now what was the variety of the bus?

Word that within the alternate, the “no” uttered by Wizard A doesn’t imply that he refuses to reply however that the data of his age and the variety of his youngsters doesn’t make it potential to know their particular person ages. After all, it’s important to assume that Wizard B is aware of the variety of the bus. Word, too, that the wizards could be very younger or very previous: Wizard A may properly be two or 20,000 years previous.

Right here is the answer, which I used to be solely in a position to totally perceive and confirm with the assistance of a program. I can’t reproduce all of the calculations required for the conclusion, however you’ll be able to take my phrase for it —or redo all of the calculations I’ve overlooked.

Allow us to denote the age of Wizard A as *a*, the variety of the bus as *b* and the variety of Wizard A’s youngsters as *c*. Suppose, for instance, that the variety of the bus is *b* = 5. The next are the choices for the variety of youngsters, the distribution of their ages and the age of Wizard A:

*c*= 5 of ages 1, 1, 1, 1, 1; thus,*a*= 1;*c*= 4 of ages 1, 1, 1, 2; thus,*a*= 2;*c*= 3 of ages 1, 1, 3; thus,*a*= 3;*c*= 3 of ages 1, 2, 2; thus,*a*= 4;*c*= 2 of ages 1, 4; thus,*a*= 4;*c*= 2 of ages 2, 3; thus,*a*= 6;*c*= 1 of age 5; thus,*a*= 5.

In every case, figuring out the age of Wizard A and the variety of youngsters signifies the potential ages of the latter. As a result of Wizard A answered “no” and Wizard B is aware of the variety of the bus, this implies *b* just isn’t equal to five.

Equally, you’ll be able to resolve the issue by inspecting the potential bus numbers one after the other to seek out these for which figuring out the age of the wizard and the variety of his youngsters won’t allow you to know the ages of every of the youngsters (a property we denote as P). Calculating *b* = 1, 2, 3, …, 12 (as we now have simply accomplished for *b* = 5) exhibits that *b* = 12 is the smallest quantity possessing P.

Certainly, for *b* = 12 and *c* = 4, there are two units of potential ages of the 4 youngsters—(2, 2, 2, 6) and (1, 3, 4, 4)—which each give the identical age for Wizard A: *a* = 48. No different two units of the identical size with the identical product for *b* = 12 exist. Subsequently, for *b* = 12, even figuring out that *c* = 4 and *a* = 48, it’s unattainable to infer the ages of the 4 youngsters. Does this imply that *b* = 12 is the answer?

Sadly, not but. For bus quantity *b* = 13, for instance, as a result of the 2 potential units of the three youngsters’s ages—(1, 6, 6) and (2, 2, 9)—are each suitable with *a* = 36, Wizard B can not derive the ages of Wizard A’s youngsters from the data of both his age or the variety of his youngsters.

Realizing that *b* = 12 isn’t any extra conclusive concerning the ages of the youngsters than figuring out that *b* = 13. When confronted with the puzzle, most individuals usually reply “*b* = 12,” as if the riddle someway implies that the smallest potential resolution for *b* is the appropriate one. However the puzzle doesn’t make that assertion. Furthermore, with out additional reasoning, you can’t select between *b* = 12 and *b* = 13, nor are you able to select amongst different values of *b*, as my additional calculations present.

But *b* = 12 is the right reply, and the rationale for that’s the most attention-grabbing and sudden a part of the riddle. Conway crafted his puzzle fastidiously, and you need to take into account the ultimate assertion of Wizard B. Following Wizard A’s “no,” Wizard B responds, “Aha! AT LAST I understand how previous you might be!” That eliminates *b* = 13.

In actual fact, for *b* = 13, there are two extra units of the youngsters’s ages—(1, 2, 2, 2, 6) and (1, 1, 3, 4, 4)—that give *a* = 48. In different phrases, if the bus quantity had been 13, Wizard B couldn’t deduce the age of Wizard A from his damaging reply as a result of his age could possibly be 36 or 48. Thus, *b* = 13 ought to be eradicated.

But eliminating *b* = 13 results in the elimination of *b* = 14 once we take into account the age sequences discovered for *b* = 13 and add 1. Doing so exhibits that the product of two units of the youngsters’s ages—(1, 1, 6, 6) and (1, 2, 2, 9)—is *a* = 36, whereas the product of one other two units—(1, 1, 2, 2, 6) and (1, 1, 1, 3, 4, 4)—is *a* = 48. The identical course of eliminates *b* = 15 and, one after the other, the entire *b*’s better than 12.

Consequently, it’s only bus quantity 12 (*b* = 12), with two units of the youngsters’s ages—(2, 2, 2, 6) and (1, 3, 4, 4)—that uniquely determines the age of Wizard A: *a* = 48.

Given all of the calculations you’d must do to reach on the resolution—the main points of which I’ve not reproduced right here and which take up a number of pages—I confess that I don’t perceive how Conway managed to conceive this unbelievable puzzle!

## Paving a Aircraft with Line Segments

Paving a aircraft with squares is straightforward. It’s simply as straightforward to take action utilizing equilateral triangles or hexagons. It is usually potential to pave the aircraft with infinite straight strains: simply place all of them facet by facet and parallel. There might be an infinite variety of them, however the paving might be completely passable as a result of every level of the aircraft will belong to 1 —and just one—line of the paving. Listed here are just a few trickier questions:

- Can the aircraft be paved with closed segments—i.e., straight line segments [A, B], together with their endpoints?
- Can the aircraft be paved with open segments—i.e., segments ]A, B[, without their endpoints?
- Can the plane be paved with semi-open segments—i.e., segments ]A, B], with solely one in all their endpoints?

Take into consideration these uncommon but completely pure questions. They aren’t so easy. Conway and a colleague mentioned them and different kinds of issues in a splendid paper printed in 1964, proving as soon as once more that quite simple questions that nobody thinks about are nice alternatives to do arithmetic that’s not so apparent.

In that paper, Conway and mathematician Hallard Croft devised a technique to fill a aircraft with straight line segments, which could be seen in “Fantastic Paving” under.

Panel *a* exhibits find out how to pave a aircraft with semi-open segments through which one level is included and one other is excluded. Doing so is straightforward as a result of inserting them collectively, head to tail, reproduces the apparent paving with straight strains.

Panel *b* exhibits the answer for equal closed segments through which each factors of every section are included. Pillar 1 is positioned first. Pillar 2 is then added, however in fact the leftmost section of this stack just isn’t retained. For pillar 3, the rightmost section just isn’t retained. Ever finer slanted pillars are added successively, omitting the “first” section for every.

Panel *c *exhibits an answer for in another way sized open segments (with their endpoints excluded). The segments create a central sq. open on all sides—i.e., neither the edges nor the highest or backside of the sq. are coated. Every successively added rectangle can also be open. For open segments of the identical size, there is no such thing as a resolution for paving the aircraft.

## The Collatz Conjecture

One other puzzle is as attention-grabbing to beginner mathematicians as it’s to professionals. Think about a operate (*f*) that provides *n*/2 for constructive integers (*n*) which can be even and three*n* + 1 for these which can be odd. For instance, beginning with 7 and making use of *f*, you get 22, then 11, 34, 17, 52, 26, 13, 40, 20, 10, 5, 16, 8, 4, 2, 1, 4, 2, 1, and so forth. When you get to 1, you go round in circles.

No matter integer *n* you begin with, you at all times appear to finish up at 1 after which get caught in a loop. Laptop calculations have examined this property, which is true for all *n*’s as much as at the very least 87 x 2^{60} (about 10^{20}). It has not been proved, nevertheless, that that is at all times the case, nor has an preliminary *n* been discovered that will proceed to infinity or result in a cycle aside from 4, 2, 1.

This downside is named the Collatz conjecture or the Syracuse conjecture. A lot work has been dedicated to it, a few of which has been compiled in a ebook. The nice simplicity of the issue’s assertion attracts amateurs, and I recurrently obtain proposed options that, to this point, are both flawed or incomprehensible.

Confronted with such a problem—so merely said and but unsolved, which remains to be the case after greater than 80 years—Conway couldn’t resist.

In 1972 he printed his first paper on the topic, through which he proposed equally formulated issues and demonstrated their undecidability. For sure values of the beginning integer, the sequence generated by the variant of *f* doesn’t find yourself arriving at 1, however set concept can not display this. Extra typically, for any logical system of demonstration (S), there’s a downside of this class and a place to begin for a sequence that doesn’t find yourself at 1, which can’t be demonstrated by S.

In 2013 Conway returned to the issue with probabilistic arguments suggesting that the Collatz conjecture itself is unprovable with the axiom methods which can be normally utilized in arithmetic. This was not a definitive demonstration of the undecidability of the Collatz conjecture. However the kinds of arguments he proposed appeared to strongly point out that it’s no accident that everybody stumbles of their try to show the conjecture.

All mathematicians encounter issues they can’t resolve, and Conway was no exception to this normal rule. However, his ability as a logician might have afforded some consolation by enabling him to display undecidability and to elaborate arguments suggesting that this easy but intractable downside would proceed to problem mathematicians indefinitely.

## A New Sample within the Sport of Life

The Sport of Life, first launched in Martin Gardner’s Mathematical Video games column in *Scientific American* in 1970, remains to be being studied, and the entire puzzles that it poses haven’t been solved. A sq. cell in an infinite, two-dimensional rectangular grid could also be alive or lifeless. Or the cell might go from dwell (*black*) to lifeless (*white*), or vice versa, from one era to a different. Or the cell might stay steady, relying on whether or not its eight nearest neighbor cells are lifeless or alive.

The rule of evolution could be expressed in 12 phrases: start if three dwell neighbors; survival if two or three dwell neighbors. Some patterns with few dwell cells on the preliminary state develop to infinity. Ideally, the variety of dwell cells will increase proportionately with *n*^{2}, the place *n* is the variety of the era. The optimum density of dwelling cells in a steady a part of the grid is 1/2. Noam Elkies of Harvard College demonstrated the proof in 29 pages in 1997.

The extraordinary sample proven on the prime of “Sport of Life Redux” was found in April 2020 by pc scientist Mateusz Naściszewski. It’s the smallest recognized sample that, as soon as launched, grows quadratically to cowl the grid of a steady dwell cell inhabitants of density 1/2. (This sample is thus the very best one by way of velocity and density.) The configuration begins with a inhabitants of 183 cells. We drew era 0 and two different generations at completely different scales [*see **bottom patterns in “Game of Life Redux”*]. It’s believed to be unattainable to do higher than 183, however that has not been demonstrated. Word, too, that there are patterns that calculate the prime numbers or that even show graphic pictures of decimal digits of π = 3.14159… one after the opposite.** **

*This text initially appeared in *Pour la Science* and was reproduced with permission.*