Appendix: Generators and relations First courses in group theory traditionally take a student 'as far as' the classifica- ti on theorem for finitely generated abelian groups, but invariably omit any discussion of free groups, or of the idea of presenting a group by means of generators and relations. Since these latter ideas are particularly important in topology (most especially for us in Chapters 6 and 10), we ofTer aquick survey here. Perhaps the easiest idea to understand is that of a free set of generators for a given group. A subset X of a group G is called a free set of generators for G if every g E G - {e} can be expressed in a unique way as a product (*) of finite length, where the Xi lie in X, Xi is never equal to Xi + hand each ni is a nonzero integer. We call the set of generators free because by the uniqueness of (*) there canbe no relations between its elements. If G has a free set of generators, then it is called a free group. Given a nonempty set X, we can construct ourselves a group which has X as a free set of generators as folIows. Define a word to be a finite product xi! ... Xk k in which each Xi belongs to X, and the n i are all integers, and say that the word is reduced if Xi is never equal to Xi + 1 and all the ni are nonzero. Given any word, we can make a reduced word out of it by collecting up powers when adjacent elements are equal, and omitting zeroth powers, continuing this process several times if necessary. An example is worth a page of explanation: x1 3 xi xis xi = x1 1 xi = X1 1 xi = which is now reduced. Reducing the word X? gives a word with no symbols which we refer to as the empty word. Now we can multiply words together simply by writing one after the other. If we do this with reduced words, the product may not be reduced, but it does simplify down to a well-defined reduced word which we call the product of the two given reduced words. The set of all reduced words forms a group under this multiplication (of course there is a lot of rather 241
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Appendix: Generators and relations
First courses in group theory traditionally take a student 'as far as' the classificati on theorem for finitely generated abelian groups, but invariably omit any discussion of free groups, or of the idea of presenting a group by means of generators and relations. Since these latter ideas are particularly important in topology (most especially for us in Chapters 6 and 10), we ofTer aquick survey here.
Perhaps the easiest idea to understand is that of a free set of generators for a given group. A subset X of a group G is called a free set of generators for G if every g E G - {e} can be expressed in a unique way as a product
(*)
of finite length, where the Xi lie in X, Xi is never equal to Xi + hand each ni is a nonzero integer. We call the set of generators free because by the uniqueness of (*) there canbe no relations between its elements. If G has a free set of generators, then it is called a free group.
Given a nonempty set X, we can construct ourselves a group which has X as a free set of generators as folIows. Define a word to be a finite product xi! ... Xkk
in which each Xi belongs to X, and the ni are all integers, and say that the word is reduced if Xi is never equal to Xi + 1 and all the ni are nonzero. Given any word, we can make a reduced word out of it by collecting up powers when adjacent elements are equal, and omitting zeroth powers, continuing this process several times if necessary. An example is worth a page of explanation:
x1 3 xi x~ xis xi x~ = x11 x~ xi x~
= X1 1 xi x~
= X~ X~
which is now reduced. Reducing the word X? gives a word with no symbols which we refer to as the empty word. Now we can multiply words together simply by writing one after the other. If we do this with reduced words, the product may not be reduced, but it does simplify down to a well-defined reduced word which we call the product of the two given reduced words. The set of all reduced words forms a group under this multiplication (of course there is a lot of rather
241
BASIC TOPOLOGY
tedious eheeking to be done); the identity element is the empty word, and the inverse ofthe redueed word xi1 ... Xkk is x;;nk ••• xl n1 .
We shall eall this group the free group generated by X, and denote it by F(X). It should be clear that iftwo sets have the same eardinality (in other words, if there is a one-one onto eorrespondenee between them) then the free groups generated by them are isomorphie. The free group with a single generator x is the infinite eyclie group, the only possible nonempty redueed words being the powers xn•
Very often one says that a given group is determined by a set of generators and a set of relations. For example, we may say that the dihedral group with 10 elements is determined by two generators x, y subjeet to the relations XS = e, y2 = e, xy = yx- l. We have in mind an intuitive idea that all the elements of the group ean be built as produets of powers of x and y, and that the multiplieation table of the group is eompletely specified by the given relations. We shall now make this precise using the notion of a free group.
Let G be a group, and X a subset whieh generates G. There is a natural homomorphism from the free group F(X) onto G whieh sends a redueed word xi1 ••• X;:k onto the eorresponding produet of group elements in G (again we omit the details); it is onto beeause X generates G. If N denotes the kernel of this homomorphism, then F(X)/N is isomorphie to G; so N determines G. Now let R be a eolleetion ofwords in F(X) with the property that N is the smallest normal subgroup eontaining them. These words, together with all their eonjugates, generate N, and they determine exaetly which words in F(X) beeome the identity when we pass from F(X) to G; that is to say, whieh produets of elements of Gare the identity in G. In this situation, we say that the pair X, R is a presentation for the group G. If Xis a finite set, with elements Xl" •. ,xm, and R is a finite set ofwords, with elements rl, ... ,rn, we say that Gis finitely presented and write
Examples 1. Z = {x 10} 2. Zn = {x I xn} 3. The dihedral group with 2n elements is
D2n = {x,y I xn,y2, (xy)2}
4. Z x Z = {x,y I x Y x-l y-l}
We finish with abrief mention of free produets. If G and H are groups we ean form 'words' XIX2" 'Xn> where eaeh Xi lies in the disjoint union G u H. Call a word reduced this time if Xi and Xi + 1 never belong to the same group, and if Xi is never the identity of G or H. Throw in the empty word, multiply redueed words by juxtaposition, reducing the produet as neeessary, and the
242
APPENDIX
result is a group called the free product G '" H of G and H. In this book, we only have occasion to take the free product of groups which are finitely presen ted, and we note that if
then
We note also that the free product 7L '" 7L '" •.• * 7L of n copies of the infinite cyclic group is just the free group on a set of size n.
The most important facts concerning free groups and free products are the following characterizations, which we give without proof: (a) Let X be a sub set of a group G. Then, Xis a free set of generators for
G iff given an arbitrary group K, plus a funetion from X to K, there is a unique extension of this funetion to a homomorphism from all of G to K.
(b) Let P be a group which eontains both G and H as subgroups. Then Pis isomorphie to the free produet G * H, via an isomorphism whieh is the identity on both G and H, iff given an arbitrary group K, plus a homomorphism from eaeh of G and H to K, there is a unique extension of these homomorphisms to a homomorphism from all of P to K.
243
Bibliography
Tbree classics [1] Hilbert, D. and S. Cohn-Vossen, Geometry and the Imagination, Chelsea, New York,
1952. [2] Lefschetz, S., Introduction to Topology, Princeton, 1949. [3] Seifert, H. and W. Thre1fall, Lehrbuch der Topologie, Teubner, Leipzig, 1934;
Che1sea, New York, 1947.
Books at about tbe same level [4] Agoston, M. K., Algebraic Topology: A First Course, Marcel Dekker, New York,
1976. [5] Blackett, D. W., Elementary Topology, Academic Press, New York, 1967. [6] Chinn, W. G. and N. E. Steenrod, First Concepts of Topology, Random House,
New York, 1966. [7] Crowell, R. H. and R. H. Fox, Introduction to Knot Theory, Ginn, Boston, 1963;
Springer-Verlag, New York, 1977. [8] Gramain, A., Topologie des Surfaces, Presses Universitaires de France, Paris, 1971. [9] Massey, W. S., Algebraic Topology: An Introduction, Harcourt, Brace and World,
1967; Springer-Verlag, New York, 1977. [10] Munkres, J. R., Topology, Prentice Hall, Englewood Cliffs, N.J., 1975. [11] Singer,1. M. and J. A. Thorpe, Lecture Notes on Elementdry Topology and Geometry,
Scott Foresmann, Glenview, Ill., 1967; Springer-Verlag, New York, 1977. [12] Wall, C. T. C., A Geometrie Introduction to Topology, Addison Wesley, Reading,
Mass., 1972. [13] Wallace, A. H., An Introduction to Algebraic Topology, Pergamon, London, 1957.
More advanced texts [14] Hilton, P. J. and S. Wylie, Homology Theory, Cambridge, 1960. [15] Hirsch, M. W., Differential Topology, Springer-Verlag, New York, 1976. [16] Hocking, J. G. and G. S. Young, Topology, Addison Wesley, Reading, Mass., 1961. [17] Kelley, J. L., General Topology, Van Nostrand, Princeton, N.J., 1955; Springer-
Verlag, New York, 1975. [18] Maunder, C. R. F., Algebraic Topology, Van Nostrand Reinhold, London, 1970. [19] Milnor, J., Topology from the Differentiable Viewpoint, University of Virginia
Press, Charlottesville, 1966. [20] Rolfsen, D., Knots and Links, Publish or Perish, Berkeley, 1976. [21] Rourke, C. P. and B. J. Sanderson, Piecewise Linear Topology, Springer-Verlag,
Berlin, 1972. [22] Spanier, E. H., Algebraic Topology, McGraw-Hill, New York, 1966.
244
BIBLIOGRAPHY
Papers [23] Bing, R. H., 'The e1usive fixed point property', Amer. Math. Monthly, 76, 119-132,
1969. [24] Doy1e, P. H., 'Plane separation', Proc. Camb. Phi!. Soc., 64, 291, 1968. [25] Doy1e, P. H. and D. A. Moran, 'A short proof that compact 2-manifolds can be
triangulated', Inventiones Math., 5, 160-162, 1968. [26] Tucker, A. W., 'Some topological properties of disc and sphere', Proc. 1st Canad.
Math. Congr., 285-309, 1945.
History [27] Pont, J. c., La Topologie Algebraique des Origines a Poincare, Presses Univer
sitaires de France, Paris, 1974.
Algebra [28] Hartley, B. and T. O. Hawkes, Rings, Modules and Linear Algebra, Chapman and
Hall, London 1970. [29] Lederman, W. Introduction to Group Theory, Longmans, London, 1976.
Comments [1] is unbeatable for sheer enjoyment and has a chapter on e1ementary topology. Massey [9] is particularly good for surfaces, van Kampen's theorem, and covering spaces; his approach is different from ours, and his applications mainly directed towards proving results in group theory. Yet another way of classifying surfaces is provided by Gramain's elegant treatment in [8]. A1gebraic topology at this level is nicely presented in [4] and [13], the first being particularly strong on applications and background history, and the second providing a contrasting approach with an account of singular homology.
Turning to more advanced material, for point-set topology [10], [16]," and Kelley's classic [17] are very good indeed. In algebraic topology, the exact sequences ofhomology, cohomology, and duality are the next topics to look for. Of [14], [18], [22], Maunder is probably the easiest to break into. Finally, for topology with a more geometrical flavour we recommend [15], [20], [21], and especially [19].
245
Index
Abelianized knot group, 222 Accumulation point (= limit point), 29 Action of group on space, 79
fixed point free, 158 simplicial, 141
Adding a handle, 16, 149 Addition of knots, 225 Alexander polynomial, 237 Annulus, 7 Antipodal map, 80, 91, 199
Face of simplex, 120 Figure of eight knot, 213 Finite complement topology, 14, 29 Finite simplicial complex, 121 Finitely presented group, 242 First homology group, 175
relation with fundamental group, 182 Fixed point free group action, 158 Fixed point free homeomorphisms of Sn, 197 Fixed point property, 111 Fixed point theorem:
of Brouwer, 110, 191 of Lefschetz, 207
Flow, irrational, 83 Flow line, 83 Folding map, 36 Free group, 242 Free product, 243 Free set of generators, 241 Frontier, 30 Fundamental group, 93
as invariant of homotopy type, 106 change of base point, 94 of bouquet of cireles, 136, 147 of cirele, 96 of elose.d surface, 168 of complement of a knot, 221 of Klein bottle, 101, 137, 138 of Lens space, 100 of orbit space, 147 of pn , 100 of polyhedron, 133 of product space, 101 of Sn, 99, 131, 136 of torus, 100 van Kampen's theorem, 138
Fundamental region, 84 Fundamental theorem of algebra, 109
General linear group, 74, 76 General position, 119, 120 Generator, 242 Genus:
of elosed surface, 169 of compact surface, 170 of knot, 224
as invariants of homotopy type, 189 of dosed surface, 183 of cone, 181 ofS·, 181, 182 with integer coefficients, 178 with rational coefficients, 200 with Z2 coefficients, 203
Homotopic maps, 88 Homotopy, 88
null homotopic, 91 relative to a subset, 88 straight line, 89
Homotopy dass, 92 Homotopy equivalence, 103 Homotopy-lifting lemma, 98, 228 Homotopy type, 103 Hopf trace theorem, 207 House with two rooms, 109 Hyperboloid, one sheeted, 7 Hyperplane, 119
Underpass, 216 Unit ball, 36 Unit cube, 36 Unit disc, 29 Universal covering space, 232 Universal television aerial, 146
Van Kampen's theorem, 138 Vector field, 198
on sphere, 198 on torus, 198
Vertex, 120 Vertex scheme, 140
Wild knot, 215 Word,241
empty,241 product,241 reduced, 241
Zz coefficients for homology, 203 Zeroth homology group, 180
INDEX
251
Undergraduate Texts in Mathematics
Icontinuedfrom page ii)
Halmos: Finite-Dimensional Vector Spaces. Second edition.
Halmos: Naive Set Theory. HämmerlinJHoffmann: Numerical
Mathematics. Readings in Mathematics.
HarrislHirstlMossinghoff: Combinatorics and Graph Theory.
Hartshorne: Geometry: Euclid and Beyond.
Hijab: Introduction to Ca1culus and Classical Analysis.
HiltonJHoltonIPedersen: Mathematical Reflections: In a Room with Many Mirrors.
HiltonJHoltonIPedersen: Mathematical Vistas: From a Room with Many Windows.
Iooss/Joseph: Elementary Stability and Bifurcation Theory. Second edition.
Isaac: The Pleasures ofProbability. Readings in Mathematics.
James: Topological and Uniform Spaces.
Jänich: Linear Algebra. Jänich: Topology. Jänich: Vector Analysis. Kemeny/SnelI: Finite Markov Chains. Kinsey: Topology of Surfaces. Klambauer: Aspects of Calculus. Lang: A First Course in Calculus. Fifth
edition. Lang: Calculus of Several Variables.
Third edition. Lang: Introduction to Linear Algebra.
Second edition. Lang: Linear Algebra. Third edition. Lang: Short Ca1culus: The Original
Edition of "A First Course in Calculus."
Lang: Undergraduate Algebra. Second edition.
Lang: Undergraduate Analysis.
Lax/Burstein/Lax: Calculus with Applications and Computing. Volume l.
LeCuyer: College Mathematics with APL.
Lidl/Pilz: Applied Abstract Algebra. Second edition.
Logan: Applied Partial Differential Equations.
Macki-Strauss: Introduction to Optimal Control Theory.
Malitz: Introduction to Mathematical Logic.
MarsdenIWeinstein: Calculus I, 11, III. Second edition.
Martin: Counting: The Art of Enumerative Combinatorics.
Martin: The Foundations of Geometry and the Non-Euclidean Plane.
Martin: Geometrie Constructions. Martin: Transformation.Geometry: An
Introduction to Symmetry. MiIlmanlParker: Geometry: AMetrie
Approach with Models. Second edition.
Moschovakis: Notes on Set Theory. Owen: A First Course in the
Mathematical Foundations of Thermodynamies.
Palka: An Introduction to Complex Function Theory.
Pedrick: A First Course in Analysis. PeressinilSullivanlUhl: The Mathematics
of Nonlinear Programming. Prenowitz/Jantosciak: Join Geometries. Priestley: Calculus: A Liberal Art.
Second edition. ProtterlMorrey: A First Course in Real
Analysis. Second edition. ProtterlMorrey: Intermediate Calculus.
Second edition. Pugh: Real Mathematical Analysis Roman: An Introduction to Coding and
Information Theory.
Undergraduate Texts in Mathematics
Ross: Elementary Analysis: The Theory of Calculus.
Samuel: Projective Geometry. Readings in Mathematics.
Saxe: Beginning Functional Analysis Scharlau/Opolka: From Fermat to
Minkowski. Schiff: The Laplace Transform: Theory
and Applications. Sethuraman: Rings, Fields, and Vector
Spaces: An Approach to Geometrie Constructability.
Sigler: Algebra. SilvermanlTate: Rational Points on
Elliptic Curves. Simmonds: A Brief on Tensor Analysis.
Second edition. Singer: Geometry: Plane and Fancy. SingerlThorpe: Lecture Notes on
Elementary Topology and Geometry.
Smith: Linear Algebra. Third edition.
Smith: Primer of Modern Analysis. Second edition.
StantonIWhite: Constructive Combinatorics. Stillweil: Elements of Algebra: Geometry,
Numbers, Equations. StillweIl: Mathematics and Its History.
Second edition. StillweIl: Numbers and Geometry.
Readings in Mathematics. Strayer: Linear Programming and Its
Applications. Toth: Glimpses of Algebra and Geometry.
Second Edition. Readings in Mathematics.
Troutman: Variational Calculus and Optimal Contro!. Second edition.
Valenza: Linear Algebra: An Introduction to Abstract Mathematics.
Whyburn/Duda: Dynamic Topology. Wilson: Much Ado About Calculus.