The Mathematics of Juggling

239 Pages·2003·2.17 MB·English

by  Burkard Polster (auth.)

The Mathematics of Juggling
Springer
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Burkard Polster
The Mathematics
of Juggling
With 114 Illustrations
BurkardPolster
DepartmentofMathematics
MonashUniversity
P.O.Box28M
Victoria3800
Australia
[email protected]
MathematicsSubjectClassification(2000):05-02,05A05
LibraryofCongressCataloging-in-PublicationData
Polster,Burkard.
Themathematicsofjuggling/BurkardPolster.
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Includesbibliographicalreferencesandindex.
ISBN0-387-95513-5(alk.paper)
1.Sequences(Mathematics) 2.Juggling—Mathematics. I.Title
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To My Jugglable Anu
Preface
Recenthistorysawanunprecedented riseinamateurjuggling.Today,there
arehundredsofjugglingclubsaroundtheworld,thereareanumberofspe-
cialized juggling magazines, and every year thousands of jugglers gather
at juggling conventions to practice, exchange ideas, take part in competi-
tions,andhavefuntogether.Jugglingisparticularlypopularamongmath-
ematicallymindedpeople such asmajorsincomputer science, engineering,
mathematics, and physics. In fact, although exact numbers are hard to
come by, an estimate that keeps popping up in this context is that up to
forty percent of serious jugglers today have a mathematical background;
see, for example, [118]. Given this strong interest in jugglingin the (hard)
scientific community, it is not surprising that many mathematical aspects
of jugglinghave been explored in depth, sophisticated computer programs
have been written that simulatejuggling,and a number of jugglingrobots
have even been built.
First and foremost, the target audience of The Mathematics of Juggling
consists of all mathematicallyminded jugglers. However, mathematics ed-
ucators, readers of popular mathematical literature, and mathematicians
interested in unusual applications of some of their favorite tools and tech-
niques willalso be among the readers of this book.
Also included in this book is a chapter dedicated to the mathematics
of bell ringing and new connections between bell ringing and toss jug-
gling. Furthermore, this chapter is the most comprehensive introduction
to mathematical bell ringing availablein the literature and, consequently,
many people interested in this ancient art will want to study at least this
part of the book.
viii Preface
My aims in writing this book were the following:
• Serious Mathematics. Many good books are availablethat are dedi-
cated to teaching the different juggling tricks and skills; see, for ex-
ample,[28],[40],and[94].Onthe other hand,manypopulararticles,
serious papers,andevenPh.D.theses havebeen written dealingwith
different aspects of the mathematics behind these tricks and skills.
Themaingoalofthisbookistosummarize,connect, andexpandthe
results dealing with mathematical aspects of juggling in the litera-
ture.
• Serious Juggling.Introducejugglerstonewsystematicwaysofthink-
ing about jugglingand the way they acquire new juggling skills and
tricks. First and foremost, this includes a description and analysis
of a compact mathematical language for juggling patterns that has
already led to the discovery of many new and attractive tricks.
• Mathematics Education. Dueto the abstract nature of mathematics,
mathematiciansoftenhaveahardtimecommunicatingtothegeneral
publicthatmathematicsand,inparticular,puremathematicscanbe
a lot of fun and very useful. Speaking from experience, I know that
a talk/performance of mathematical jugglingis a perfect ice-breaker
in this respect. I hope that this book willlead more scientists to use
mathematical jugglingin outreach programs and to communicate to
the general public that mathematics can be a lot of fun as well as
useful in modeling and understanding just about every problem in
real life.
• Turn Jugglers into Mathematicians. Using juggling as a unifying
theme, provide an intellectually stimulating introduction for mathe-
matically wired jugglers to many beautiful results and techniques
fromawiderangeofmathematicaldisciplinessuch ascombinatorics,
graph theory, group theory, knot theory, classical mechanics, and
number theory.
• Turn Mathematicians into Jugglers.Makepeoplewhoareabletoap-
preciate and enjoy the mathematics in this book also give the prac-
tical side of jugglinga go.
• Bell Ringing. As with juggling, the art of bell ringing has a strong
following in the scientific community, and the appeal of bell ringing
andjugglingtothescientificmindseems tobeverysimilar.However,
not many jugglers seem to know about bell ringing and, vice versa,
not many bell ringers also practice juggling. I hope that including
and linking the mathematical aspects of these seemingly unrelated
pastimes willenrich and complement the overallmathematicalmenu
offered in this book and introduce and awaken the interest of many
Preface ix
readerswhoareonlyawareofjugglingorbellringingtotherespective
other activity.
Thisbookisdesigned tobeaccessibleat different levelsofmathematical
sophistication. Anybody who is not put off from picking up this book in
the first place by the word “mathematics” in the title should be able to
get something out of it. At the same time, full proofs, or at least detailed
sketches of proofs, havebeen included for allthose people who are ableto
appreciate “serious” mathematics.
Description of Contents
In the following,I summarize the different chapters and the material cov-
ered in this book.
The first chapter consists of a brief introduction to jugglingand its his-
tory. In particular, it contains a detailed account of the development of
mathematical juggling starting around 1985. This also includes brief re-
views of a number of freely availablecomputer jugglingprograms that are
basedonthemathematicallanguageforjugglingpatterns developedinthis
book. These programsillustratevividlythepower ofthe mathematicalap-
proach to juggling by conjuring up virtual jugglers that can juggle any
conceivable number of props in infinitely many ways and all this without
drops. I highlyrecommend that youuse one or two of these programs side
by side with this book. I also take the opportunity to acknowledge the
important, yet not very well-known, contributions of many people to the
developmentofmathematicaljugglingviatheirarticlespostedtothenews-
group rec.juggling, their development of computer juggling programs,
and so forth.
InChapter2,westripawayalltheflourishesandcontortionsinvolvedin
aperiodicjugglingpatterninwhichthrowsoccuratdiscreteequallyspaced
moments in time and in which at most one ball is caught and thrown at
any given time. What we are left with can be described by a (simple) jug-
gling sequence; that is, a special finite sequence of nonnegative numbers
that records thenumber ofbeats theindividualthrows inthe pattern stay
in the air. We investigate tests based on averages, juggling diagrams, and
permutations that allow us to recognize juggling sequences and the num-
ber of balls that are used in juggling a given juggling sequence. We prove
that any finite sequence of nonnegative numbers, the sum of whose ele-
ments is divisible by the length of the sequence, can be rearranged into
a juggling sequence. Using juggling cards, we generate all b-ball juggling
sequences, derive a formula for the number of b-ball juggling sequences of
period p,and describe some close relationships between jugglingsequences
and elements ofcertain affine Weyl groups. Weintroduce state graphs and
transition matrices based on which all b-ball jugglingsequences of a given
maximal throw height can be generated. Furthermore, we derive various
results about state graphs such as upper and lower bounds for the lengths
x Preface
ofprimeloopsofmaximallengths,thefactthatthestates visitedbyajug-
gling sequence determine its throws, and the fact that the complement of
theb-ballstate graphofheighthisthe(h−b)-ballstategraphofheighth.
In Chapter 3, we consider multiplex juggling sequences, a first general-
ization ofthesimplejugglingsequences considered inChapter 2. Unlikein
simple jugglingsequences, in a multiplex jugglingsequence several throws
can be made simultaneously on every beat. We find that many of the re-
sults that we derived for simple juggling sequences have counterparts for
multiplex jugglingsequences. Also included in this chapter are several ap-
plications that illustrate how to derive, entirely within the framework of
multiplex juggling sequences, combinatorial identities involving Gaussian
coefficients, Stirling numbers of the second kind, and their q-analogues.
Finally, we include a summary of operations that enable us to transform
simple and multiplex jugglingsequences into other jugglingsequences.
All juggling sequences in Chapters 2 and 3 can, at least in theory, be
juggledusing onehandonly.In Chapter 4,weconsider multihandjuggling
inwhichseveralmultiplexthrowscanbemadesimultaneouslybyanumber
of different hands. Multihand juggling patterns are described by juggling
matrices. Again, we find that virtually all basic results for simple juggling
sequences have counterparts for juggling matrices. This includes the dif-
ferent ways to calculate the number of balls required to juggle a juggling
matrix, an algebraic test that allows one to check whether or not a given
matrixoftheappropriateformisajugglingmatrix,andresultsaboutstate
graphs ofjugglingmatrices. Wealsofindthat mostofthe usual operations
introduced for jugglingsequences have counterparts for jugglingmatrices.
However, we also encounter some new operations that allow one to “con-
tract” any juggling matrix into a juggling sequence and to systematically
construct all jugglingmatrices from jugglingsequences.
Wehaveacloserlookatanumberofdistinguishedsubclasses ofjuggling
matrices. This includes the simple juggling matrices in which every hand
handles at most one ball at a time and the distributed juggling matrices
in which, as the name suggests, on every beat at most one of the hands
does something. Cyclic juggling matrices are special distributed juggling
matrices. In a cyclicjugglingmatrix,the different hands taketurns throw-
ing the balls in cyclical order. We are particularly interested in cascades
andfountains;thatis,jugglingpatterns described bysimplecyclicjuggling
matrices in which all nonzero throws are to the same height and one such
throw is made on every beat.
Also included in this chapter are Shannon’s fundamental results about
uniform multihand jugglingpatterns. In a uniform jugglingpattern, there
is notnecessarily anunderlyingbeat onwhich allthethrows occur. Inthis
respect, these patterns do not have to be “uniform” at all. What makes
them uniform is the fact that the dwell time that any ball is held by a
hand is constant, the flight time that any ball spends in the air between
being thrown and being caught is constant, and the vacant time that any
Preface xi
hand spends empty between throwingand catching is alsoconstant. Shan-
non’s celebrated first theorem relates the dwell,flight, and vacant times of
a uniform juggling pattern with the numbers of balls and hands used in
juggling the pattern. His second and third theorems deal with the essen-
tially different ways of uniformly juggling a given number of balls with a
given number ofhands. Westate and prove Shannon’s theorems and some
corollaries and extensions of these results. In particular, we derive gener-
alizations of Shannon’s results in the framework of simple cyclic juggling
matrices.
Anotherinterestingideathatfirstpopsupintheinvestigationofuniform
juggling patterns is that we never just juggle balls. What we really do is
juggleballsandhands.Infact,itfollowsimmediatelyfromthedefinitionof
uniform jugglingthat ifwe interpret ballsas hands and hands as balls,we
arrive at a new uniform jugglingpattern. This leads to a dualityprinciple
for uniformjugglingpatterns. Wealsoinvestigatejugglingballsand hands
in the framework of simple distributed jugglingmatrices.
In the followingsection, we show that jugglingsimple jugglingmatrices
with labeled balls still yields periodic patterns.
In the final section of this chapter, we consider natural decompositions
of simple juggling sequences that arise when these juggling sequences are
interpreted as simple cyclic juggling matrices. This includes a nice appli-
cation of the cycle representations of the permutations that are associated
with simplejugglingsequences.
Chapter 5 is dedicated to practical aspects of mathematical juggling.
First, we list and describe a number of jugglingsequences that are partic-
ularly attractive from a purely visual point of view or useful for teaching
purposes. This is followed by a discussion of ways to make juggling easier
by taking away gravity in variousways or bouncing balls and descriptions
of how these simple ways of juggling have been used in constructing jug-
glingrobots. Followingthis,we modelthe 2-hand jugglingof cascades and
fountains to deduce how high and how accurately these patterns have to
be juggled. We derive a simple model for club throws, which we then use,
in conjunction with previous results, to explain variouslining-upeffects in
juggling 2-hand fountains and cascades. Finally, we summarize the main
advantages of the mathematical language for juggling patterns developed
in the previous chapters.
Chapter6isanintroductiontomathematicalbellringinganditsconnec-
tionswithjuggling.Bellringersringthechanges.Mathematicallyspeaking,
changes arepermutations of the bells that are being rung, and ringing the
changes is really ringing special sequences of permutations. We call these
sequences “ringing sequences.” It turns out that ringing sequences can be
juggled if we use balls instead of bells and that the simple juggling se-
quences thatcorrespond toringingsequences areparticularlywell-behaved
juggling sequences. Furthermore, the sequence of transitions that corre-
sponds to a ringing sequence on b bells can be interpreted in terms of site