John Baez
July 14, 2005
The Mysteries of Counting:
Euler Characteristic versus Homotopy Cardinality
We all know what it means for a set to have 6 elements, but what sort of
thing has 1 elements, or 5/2? Believe it or not, these questions have
nice answers. The
Euler characteristic of a space is a generalization of cardinality that
admits negative integer values, while the homotopy cardinality is a
generalization that admits positive real values. These concepts shed
new light on basic mathematics. For example, the space of finite sets
turns out to have homotopy cardinality e, and this explains the key
properties of the exponential function. Euler characteristic and
homotopy cardinality share many properties, but it's hard to tell if
they are the same, because there are very few spaces for which both
are welldefined. However, in many cases where one is welldefined,
the other may be computed by dubious manipulations involving divergent
series  and the two then agree! The challenge of unifying them remains open.
The Mysteries of Counting 
transparencies in PDF format.
For more information, try these papers:

"Negative sets" and Euler characteristic:

André Joyal, Regle des signes en algebre combinatoire, Comptes
Rendus Mathematiques de l'Academie des Sciences, La Societe Royale
du Canada VII (1985), 285290.

Steve Schanuel, Negative sets have Euler characteristic and dimension,
Lecture Notes in Mathematics 1488,
Springer Verlag, Berlin, 1991, pp. 379385.

Daniel Loeb, Sets with a negative number of elements, Adv. Math.
91 (1992), 6474.

Resumming divergent Euler characteristics:

William J. Floyd and Steven P. Plotnick, Growth functions on
Fuchsian groups and the Euler characteristic, Invent. Math.
88 (1987), 129.
 R. I. Grigorchuk, Growth functions, rewriting systems and Euler
characteristic, Mat. Zametki 58 (1995), 653668, 798.

James Propp, Exponentiation and Euler measure, Algebra Universalis
49 (2003), 459471. Also available as
math.CO/0204009.

James Propp, Euler measure as generalized cardinality.
Available as math.CO/0203289.

Euler characteristics and supersymmetry:

Matthias Blau and George Thompson, N = 2 topological gauge
theory, the Euler characteristic of moduli spaces, and the Casson
invariant, Comm. Math. Phys. 152 (1993), 4171.

Euler characteristics of tame spaces:

Lou van den Dries, Tame Topology and OMinimal Structures
Cambridge U. Press, Cambridge, 1998. Chapter 4.2: Euler Characteristic.

Euler characteristics of groups:

G. Harder, A GaussBonnet formula for discrete arithmetically
defined groups, Ann. Sci. Ecole Norm. Sup. 4
(1971), 409455.

JeanPierre Serre, Cohomologie des groups discretes,
Ann. Math. Studies 70 (1971), 77169.

Kenneth S. Brown, Euler characteristics, in
Cohomology of Groups, Graduate Texts in Mathematics
182, Springer, 1982, pp. 230272.

Euler characteristics of categories and ncategories:

Tom Leinster, The Euler characteristic of a category.
Available as arXiv:math/0610260.

Tom Leinster, The Euler characteristic of a category as a
sum of a divergent series. Available as
arXiv:0707.0835.

Euler characteristics of chain complexes of graded vector spaces:

Bijective proofs in combinatorics:

Complex cardinalities:

Andreas Blass, Seven trees in one, Jour. Pure Appl. Alg.
103 (1995),
121. Also available at:
http://www.math.lsa.umich.edu/~ablass/cat.html

Robbie Gates, On the generic solution to P(X) = X in distributive
categories, Jour. Pure Appl. Alg. 125 (1998), 191212.

Marcelo Fiore and Tom Leinster,
An objective representation of the Gaussian integers,
Jour. Symb. Comp. 37 (2004), 707716.
Also available as
math/0211454.

Marcelo Fiore and Tom Leinster, Objects of categories as complex numbers,
Adv. Math. 190 (2005), 264277. Also
available as
math/0212377.

Marcelo Fiore, Isomorphisms of generic recursive polynomial
types, to appear in 31st Symposium on Principles of Programming
Languages (POPL04). Also available at
http://www.cl.cam.ac.uk/~mpf23/papers/Types/recisos.ps.gz

Applications to quantum theory:

John Baez and James Dolan,
From finite sets to Feynman diagrams,
in Mathematics Unlimited  2001
and Beyond, vol. 1, eds. Björn Engquist and Wilfried
Schmid, Springer, Berlin, 2001, pp. 2950.
Also available as
math/0004133.

Jeffrey Morton,
Categorified algebra and quantum mechanics,
TAC 16 (2006), 785854.
Also try these
expository treatments in This
Week's Finds in Mathematical Physics:
 Week 147 
Categorification, Euler characteristic versus homotopy
cardinality, and the cardinality of the fundamental group
of a Riemann surface.

Week 184 
qdeformed cardinalities: how cardinalities of Grassmannians
over finite fields relate to Euler characteristics of Grassmannians
over R and C.

Week 185 
Vector spaces over the field with q elements
as a qdeformation of finite sets; more on qdeformed cardinalities.

Week 186 
The qpolynomial of a simple algebraic group, and using it
to compute the cardinalities of flag manifolds over finite
fields, and their Euler characteristics over R and C.

Week 187 
The qpolynomial of a simple algebraic group  the classical
groups treated in detail.

Week 202 
Joyal's species and reasoning with complex cardinalities.

Week 203 
An object whose cardinality is the golden ratio.

Week 244 
Leinster's work on the Euler characteristic of a category.
For much more, try these course notes:

John Baez and Derek Wise,
Quantization and Categorification.
Finally, here's an earlier talk on this subject:
The imaginary expression √a and the negative expression
b resemble each other in that each one, when they seem the solution
of a problem, they indicate that there is some inconsistency or
nonsense.  Augustus De Morgan
Mathematics is not the rigid and rigidityproducing schema that the
layman thinks it is; rather, in it we find ourselves at that meeting
point of constraint and freedom that is the very essence of human
nature.  Hermann Weyl
© 2005 John Baez
baez@math.removethis.ucr.andthis.edu