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1 University of Oslo MAT2 Project The Banach-Tarski Paradox Author: Fredrik Meyer Supervisor: Nadia S. Larsen Abstract In its weak form, the Banach-Tarski paradox states that for any ball in R, it is possible to partition the ball into finitely many pieces, reassemble them using rotations only, producing two new balls of the exact size as the original ball. In its strong form, the paradox states that for any two bounded sets A, B R with non-empty interior, it is possible to partition A, move the pieces around, and end up with B. For a paradoxical decomposition of the sphere, it can be shown that 4 pieces actually is enough. This short paper aims to prove the Banach- Tarski paradox in its weak and strong forms, together with the justmentioned lower bound on the number of pieces in the decomposition. May, 2

4 Lemma 2.. If G is paradoxical and acts on X with no non-trivial fixed points, then X is G-paradoxical. Proof. Let A i, B i, g i, h i be as in Definition 2.. Using the Axiom of Choice, choose a set M such that M contains exactly one element from each G- orbit in X; then {g M : g G} is a disjoint partition of X: Certainly, g G g M = X, since M contains one element from each G-orbit. To prove disjointness, assume there exist g, h G such that g M h M. Then there are x, y M such that g x = h y. Then h g x = y, so x and y are in the same G-orbit, but by our choice of M, this means that x = y. So we have h g x = x. Since G act on X with no non-trivial fixed points, this means that h g =, and thus must be equal. Now, let A j = g A j {g M} (j =,..., n) and B j = g B j {g M} (j =,..., m) Obviously, A,..., A n, B,..., B m are disjoint subsets of X (since A i and B j are disjoint). We claim that X = n j= g j A j = m j= h j B j, proving that X is G-paradoxical. For suppose x X. Because M contains one element from each G-orbit, there exists a g G such that x g M. Since G is paradoxical, we know that g = g j a j for some a j A j. Thus x g j (a j M), and since a j M A j, we see that x n j= g j A j. The case for the B j s is of course identical. An immediate consequence of the preceding lemma is the following: Corollary 2.4. If F 2 acts on X with no non-trivial fixed points, then X is F 2 -paradoxical. Recall that SO() is the group of rotations of the sphere, or equivalently, the group of -matrices A such that det A =. Our next result will turn out to be the recipe for any paradoxical decomposition. Lemma 2.5. There are rotations A and B about lines through the origin in R generating a subgroup of SO() isomorphic to F 2, the free group of two generators.

5 Proof. Let A ± = and B ± = ± ± be our two rotations. Now, let w be a reduced word in A ±, B ± which is not the empty word I. We claim that w cannot act as identity on R, thus proving that A, B F 2, where A, B is the subgroup of SO() generated by A, B. Note first that, without loss of generality, we may assume that w ends in A ± (for if w were the empty word, then conjugation by A ± will not alter this). We claim that w = k a b 2 c where a, b, c Z and b and k is the length of the word w, in which case w cannot possibly act as identity on R. We will prove this by induction on k. The base case k = is simple. By assumption then, w = A ± : w = = 2 2 ± 2 2 ±2 2 = ±2 2 Certainly, k = is fine. Now, let w = A ± w or w = B ± w where w = a k b 2, c a, b, c Z and b is not divisible by. An easy calculation shows that w = k A ± { }} { 2 2 ± 2 2 a b 2 c = k = a 4b 2(b k ± 2a ) = a c k b 2. c 4 ±a 2 2. a 4b + b 2 c

6 And similarly for w = B ± w. We have: { a = a 4b, b = b ± 2a, c = c if w = A ± w a = a, b = b 2c, c = c ± 4b if w = B ± w () Obviously, a, b, c Z. We must show that b, and we will be done: Case w = A ± B ± v (where possibly v = I). We then have w = A ± B ± v so that by (), we have b = b ± 2a = b ± 6a. Since, by assumption, b, it follows that b. Case 2 w = B ± A ± v. Proven as Case. Case w = A ± A ± v. By assumption, v = k 2 a b 2 c where a, b, c Z and b. By () it follows that b = b ± 2a = b ± 2(a 4b ) = b + b ± 2a 9b = 2b 9b so b. Case 4 w = B ± B ± v. This case is treated as above. Our weak goal is to show that S 2 is SO()-paradoxical. That is, you can partition the sphere such that rotations on the pieces produce two new copies of S 2. The next result shows that this almost can be done. Lemma 2.6 (Hausdorff Paradox). There is a countable subset D of S 2 such that S 2 \D is SO()-paradoxical. Proof. Let A and B be as in the preceding proof. Let G = A, B F 2, so G is paradoxical. Since A and B are rotations about the origin, each w G\{I} has exactly two fixed points. Let D be the collection of all 5

7 points in S 2 that are fixed by some w G\{I}. Since G is countable, so is D. Clearly, G acts on S 2 \D without non-trivial fixed points: for if p S 2 \D and g G and g p = p, we would have p D which is impossible. It follows by Lemma 2. that S 2 \D is G-paradoxical. Since G SO(), we conclude that S 2 \D is SO()-paradoxical. Any rectangle can be cut in two and moved around to yield an isosceles triangle. We say that the rectangle and the isosceles triangle are equidecomposable. This notion can be generalized to sets which are acted upon by groups. Definition 2.7. Suppose G is a group acting on a set X, and that A, B X. Then A and B are called G-equidecomposable (we write A G B) if there exist subsets A i of A and B i of B such that n n A = A i = A, B = B i, (2) i= and A i A j = = B i B j for i j, and g i G such that g i A i = B i for each i {, 2,..., n}. When it is clear from the context which group we are talking about, we will write A B in place of A G B. Later, we will use the notation A n B when A and B are equidecomposable using n pieces. It will be important for us to be sure that G actually is an equivalence relation, and thus partitions P(X), the power set of X. Lemma 2.8. G is an equivalence relation on the subsets of P(X). Proof. Obviously, A G A. Just let the identity of G act on A itself. So G is reflexive. Now, assume A G B. We must show that also B G A. Since A G B, there exists a partition of A and B such that g i A i = B i for each cell in the partition. But then we have that A i = gi (g i A i ) = gi B i for each cell in the partition. Thus B G A. So G is symmetric. To show transitivity, assume A G B and B G C. First, observe that if A B =, then ga gb =. For if not, there would exist a A and b B such that ga = gb, but this implies that a = b, which is impossible (since A B = ). Now, since A G B, there is a partition of A = n i= A i and B = n i= B i such that g i A i = B i for i n and g i G. And since B G C there is a partition of B = m j= Bj and C = m j= C j such that h j B j = C j for j m and h i G. Now, observe that since m B i = ( B j ) m B i = (B i B j ), j= 6 j= i=

8 we have g i A i = m j=(b i B j ), which, by our observation, implies that { g i (B i B j ) i n, j m } is a disjoint partition of A. Thus, since B j = n i= (B i B j ), it follows that h j g i (g i n i= (B i B j )) = C j. We conclude that A G C. Notice that in the above proof, we used at most nm pieces. An easy - but nonetheless very important - observation about this relation is the following: Lemma 2.9. If A A 2 = = B B 2 with A B and A 2 B 2, then A A 2 B B 2. Proof. A B means that B = g i A i for some partition of A and g i G, and similarly B 2 = g j A j 2. Then, since A A 2 =, we can use the same partition of A A 2 as in A, A 2. The result follows. Equidecomposability makes it easier to determine if a set is paradoxical: Lemma 2.. Suppose G is a group acting on a set X and that E and E are G-equidecomposable subsets of X. If E is G-paradoxical, so is E. Proof. Assume E is G-paradoxical. Then there are A i, B i E and g i, h i as in Definition 2. where A = n A j and B = j= Where n j= g i A i = E and m j= h j B j = E. Then A G E and B G E. Since G is an equivalence relation, it is transitive, so since E G E, we have A G E and B G E. But from the definition of equidecomposability, this implies that there exist a partition A j of A and group elements k i such that E = j k J A j, and also a partition B j of B and group elements k j such that E = j k J B j. But this means that E is paradoxical. The next result shows that if you remove any countable collection of points from the sphere, you can always restore it using rotations. m j= B j 7

9 Lemma 2.. If D is a countable subset of S 2, then S 2 SO() S 2 \D. Proof. Let L be a line through the origin such that L D =. Now, define as follows: ρ is a rotation through L by the angle nθ W = θ [, 2π) for some integer n such that if x D ρ x D Since D is countable, so is W. It follows that there is an element α [, 2π)\W. Let ρ be the rotation about L through the angle α. By the definition of W, it follows that ρ n D D = for all integers n N. It follows that ρ n D ρ m D = (n, m N, n m). () Now, let D = n= ρn D. Notice that, by (), ρ D\ D = D. Then S 2 = D (S 2 \ D) ρ D (S 2 \ D) = S 2 \D, as desired. Notice the use of Lemma 2.9. Corollary 2.2. S 2 is SO()-paradoxical. Proof. Combining Lemmas 2.6, 2. and 2. immediately gives us the result: Lemma 2.6 tells us that there exists a countable subset D S 2 such that S 2 \D is SO()-paradoxical. Lemma 2. tells us that S 2 \D SO() S 2 and combined with Lemma 2., we have that S 2 is SO()-paradoxical. Let O denote the set of all Euclidean isometries preserving handedness, that is, the set of all translations and rotations. The weak Banach-Tarski paradox is a slightly stronger statement than Corollary 2.2. Lemma 2. (Weak Banach-Tarski Paradox). Every closed ball in R is O -paradoxical. Proof. Without loss of generality, we prove only that B [, R ], the closed unit ball, is paradoxical. The proof for any other ball is identical but more messy. We show first that B [, R ]\ {} is paradoxical. Since S 2 is SO()-paradoxical, there are A,..., A n, B,..., B m S 2 R and g,..., g n, h,..., h m SO() as in Definition 2.. Let A j = {tx t (, ], x A j } (j =,..., n) 8

10 and B j = {tx t (, ], x B j } (j =,..., m) We note that A i A j = when i j and that B [, R ]\ {} = n g j A j = j= m h j Bj It follows from the definition of paradoxality that B [, R ]\ {} is indeed paradoxical. Now, let x = (,, 2 ) B [, R ]\ {}. Let ρ be a rotation of infinite order about a line through x not through the origin. Let D = {ρ n n N }. Obviously, ρ D = D\ {}. Then, by earlier results: B [, R ] = D (B [, R ]\D) ρ D (B [, R ]\D) = B [, R ]\ {} It follows by Lemma 2. that B [, R ] is SO()-paradoxical, and SO() is a subgroup of O. j= An almost identical proof gives us the following: Corollary 2.4. R is paradoxical. We want however more surprising mathematics. On our road to the strong form of the Paradox, we need the following very important observation: Lemma 2.5. If A G B, then there is a bijection f : A B such that C f(c) whenever C A. Proof. Since A B, there is a partition of A = n i= A i and a partition of B = n i= B i such that g i A i = B i for g i G. Now, define f : A B as follows: f(x) = g i x if x A i Since A B, this function is onto, and it is clearly since it is defined by group actions. Thus f is a bijection. To prove the latter part of the statement, assume C A. Then A i C gives us a partition of C. Since f(c) = n i= g i (A i C), it follows by the definition of equidecomposability that C f(c). We define a relation as follows: 9

11 Definition 2.6. Let G be a group acting on a set X, and let A, B X. If A G B where B B, we write A G B. As with G, we will write A B in place of A G B when this simplification does not cause any confusion. Obviously, G is reflexive. To prove transitivity, assume A B and B C. Then A B for some B B. By Lemma 2.5, B f(b ) C, so A B f(b ) C. Thus A C. We have, in fact, that if A B and B A, then A B. In other words, is an antisymmetric binary relation. Thus defines a partial ordering on the -classes of P(X). Theorem 2.7 (Banach-Schröder-Bernstein Theorem). Suppose G is a group acting on a set X. If A, B X, A G B, B G A, then A G B. Proof. Let f : A B ( B) and g : B A ( A) be bijections as guarenteed by Lemma 2.5. Let C = A\A and define inductively C n+ = gf(c n ) where g f : A A, and let C = n= C n. We claim that A\C = g(b\f(c)). So assume x B\f(C). Then we can t have g(x) C n for any n >, for that would imply that x f(c n ). Thus g(b\f(c)) A\C. Now, note that A\C = A \C. So assume x A \C. Then there exists a y B such that g(y) = x. If y f(c), we would have g(y) = x C, which is impossible, so y B\f(C). Thus A\C g(b\f(c)). So A\C = g(b\f(c)). By our definition of g, this means that A\C B\f(C). In the same way, C f(c). So A = (A\C) C (B\f(C)) f(c) = B by Lemma 2.9. The Banach-Schröder-Berstein theorem gives us a characterization of paradoxical sets in terms of equidecomposability: Lemma 2.8. Let G be a group acting on a set X, and let E X. Then E is G-paradoxical if and only if there exist disjoint A, B such that A B = E and A E B. Proof. We already know that if E is G-paradoxical, then we can find disjoint A, B such that A E B. It remains to show that we can find A, B such that A B = E and such that the properties of A, B still holds. Let f : E A be a bijection as guaranteed by Lemma 2.5. Then, since f(e\b) A and f(e\b) E\B, we see that E\B A, so E\B E. And since E A E\B, we have E E\B. Thus, by the Banach- Schröder-Bernstein theorem, we have E E\B. So choose A = E\B and B = B.

12 The reverse implication is true by the definition of paradoxality. We have finally developed enough machinery to present the strong form of the paradox: Theorem 2.9. If A and B are bounded subsets of R, each having nonempty interior, then A B. Proof. By the Banach-Schröder-Bernstein theorem, it is enough to show that A B, for a symmetric argument produces the opposite relation. Since A is bounded, we may choose a ball K such that A K. Since B has non-empty interior, we may choose a ball L such that L B. For some n large enough, K may be covered by n overlapping copies of L. That is, choose translations t i such that K n i= t i L. Now, choose translations t i such that t i L t j L = for i j and let S = n i= t i L. Then, using the weak Banach-Tarski paradox to copy L n times, we get L S, and in particular S L. Then A K n i=t i L S L B, which together with Lemma 2.5 establishes the claim. This completes our work on the existence of paradoxical decompositions. Minimizing the number of pieces The rest of this paper will focus on minimizing the number of pieces required for a paradoxical decomposition of the sphere. Although the paradox itself may be interesting enough, knowing how many pieces suffice is interesting because of the intuitively amazing low bound on this number. In fact, the number of pieces required for a paradoxical decomposition of the sphere is 4. First, let us formally define the number r which we want to minimize: Definition.. Suppose G acts on X and E, A, B X. Then E is G- paradoxical using r pieces if A B = with A B = E such that where m + n = r. A m E n B From Lemma 2.2 we remember that F 2 is paradoxical, but in our proof we did not use all of F 2 to demonstrate paradoxality. This is however possible, as stated by the following lemma:

13 Lemma.2. Let F 2 be the free group generated by a, b. Then we can find a partition A,..., A 4 of F 2 such that A b A 2 = F 2 and A a A 4 = F 2, meaning that F 2 is paradoxical using this partition. Moreover, for any w F 2, the partition can be chosen such that w is in the same cell as the identity of F 2. This is a simple proof found in []. Proof. Suppose w is a word starting with ρ = a, say. The proof for any other ρ is identical (exchange a anywhere it occurs with ρ). Define A i as follows: (where W (ρ) denotes the set of all words starting with ρ) A = W (b) A 2 = W (b ) A = W (a)\ {a n n N} A 4 = W (a ) {a n n N} {} Now, in the same way is in the proof of Lemma 2.2, we have that F 2 = A b A 2. Showing that F 2 = A a A 4 is only slightly harder. It is clear that all powers of a are included in a A 4. So assume w is not a power of a. Then if w / W (a), a w W (a ). It follows that w = a(a w) a W (a ). Thus F 2 is paradoxical using four pieces. It is also clear that, w A 4. There is a four-piece analog to Lemma 2.: Lemma.. Let F 2 act on X without non-trivial fixed points. Then X is F 2 -paradoxical using four piees. Proof. The proof is identical to that of Lemma 2. (observe that lifting a paradox to a set preserves the number of pieces). Just use the partition of F 2 given in Lemma.2. If G acts on X and x X, let Stab(x) = {σ G σ x = x}. concept in proving the main result of this section is the following: A key Definition.4. An action of a group G on a set X is called locally commutative if Stab(x) is commutative for every x X. Equivalently, if two elements of G have a common fixed point, then they commute. We have already met locally commutative actions. Take the group of rotations of the sphere S 2 ; if two rotations share a fixed point, they must share the same axis, and thus commute. 2

14 Lemma.5. Let F 2 act on X such that the action is locally commutative. Then X is F 2 -paradoxical using four pieces. Proof. This proof is long and hard, but we ll soon get our reward. We will use Lemma.2 to partition X into A, A 2, A, A 4 satisfying b A 2 = A 2 A A 4 and a A 4 = A A 2 A 4, which suffices to prove the theorem. We will place each element of X in A i orbit by orbit (possibly using different partitions of F 2 for different orbits). Note that all F 2 -orbits in X are either composed only of fixed points or of no non-trivial fixed points: assume g x = x for x X and g F 2. If y F 2 x, then y = h x for some h F 2. Then hgh y = hgh (h x) = hg x = h x = y, so y is a fixed point. For points in orbits consisting of no fixed points, the same assignment of points is straightforward. For each orbit, fix any representative x. Then each element y in the orbit may be uniquely written as y = v x with v F 2. Now, apply Lemma.2 to partition F 2 having the additional property that and w lie in the same cell of the partition. Place y in A i if v A i F 2. For orbits consisting entirely of fixed points, we ve got to work harder. Fix an orbit O. Let w be a word of minimal length fixing a point x in O. Let ρ be the leftmost letter in w. We see that w cannot end in ρ, for if it did ρ wρ would fix ρ x and be shorter than w. We claim that we can write any point y O uniquely as v x where v does not end in w or ρ. Why? Let v be a minimal word such that y = v x. We see that v cannot end in w or w, because that would contradict v being a word of minimal length (both w and w fixes x). If however v ends in ρ, we choose vw which doesn t end in w or ρ (because ρ is the first letter in w). We will show that the only elements fixing x are powers of w. This is where local commutativity comes in. For assume u fixes x (in addition to w). Then, by local commutativity, wu = uw. But since F 2 is free, this means that w = t k and u = t l for some integers k, l and t F 2 (else we would have a non-trivial relation on the letters of F 2 ). It follows from the minimality of w that k < l. Using the division algorithm, we know that l = kn + r for some integers k, r with r < k. Thus x = u x = t l x = t kn+r x = t r (t k ) n x = t r x. Again, using the minimality of w, this yields that r =, and u is thus a power of w, as claimed.

15 The uniqueness of the representation y = v x follows easily: Assume y = v x = u x are both representations of the desired form (v should not end in w, ρ ). Then u v x = x, so one of u v or v u is a positive power of w. Assume, without loss of generality, that it is u v; then u v = w n for some integer n. Since w begins with ρ, u must begin with ρ (contradicting our assumption that u did not end in ρ ), or u cancel against v, meaning v ends in w (contradicting our assumption that v did not), and we conclude that u = v. Having established that each y O can be written uniquely as y = v x, we put y in A i if v A i F 2. To show that this assignment works, consider first the relation b A 2 A 2 A A 4. Suppose y A 2. Then y has a representation y = v x for an x in the orbit with v A 2. Now, consider b y. If bv y is the correct representation of b y, then since v A 2 implies bv A 2 A A 4, b y is properly placed in A 2 A A 4. But bv might possibly end in w or ρ. So, assume bv ends in w. Since v does not end in w, bv must equal w, so b y = w x = x. Since bv A 2 A A 4, so is w, and by our choice of partition of F 2, so is. Since x is the unique representation of x, this implies that x and hence b y = bv x = w x = x lies in A 2 A A 4. Assume now that bv ends in ρ. Since v does not, v must equal. Then y = x, bv = b and so ρ = b, and from our choice of ρ, w begins with b. Since = v A 2, by our choice of partition of F 2, w A 2 also, hence bw A 2 A A 4. Since bw x is the unique representation of b x (note that w begins with b ), we have b y = b x A 2 A A 4. An identical treatment works for the other containments b (A 2 A A 4 ) A 2, a A 4 A A 2 A 4, and a (A A 2 A 4 ) A, completing the proof. After having tortured ourselves through the preceding proof, we may proudly present the main result of this section. The sphere, S 2 is paradoxical using 4 pieces only, and if you ever try to improve that number, you won t succeed. Theorem.6. S 2 is SO()-paradoxical using four pieces, and the four cannot be improved. Proof. Since the action of rotations on the sphere is locally commutative (by the argument in the preceding paragraph), and SO() have a subgroup isomorphic to F 2, the first part of the claim follows from Lemma.5. To demonstrate that a composition in four pieces cannot be improved, assume the contrary: that S 2 contains two disjoint subsets A, B such that A m X n B where m + n < 4. At least one of m or n must equal. If 4

16 m =, then S 2 = g(a) for some g SO(), whence A = g (S 2 ) = S 2 and B =, a contradiction. And we stop here. References [] Stan Wagon, The Banach-Tarski Paradox, Cambridge University Press, 985. [2] Volker Runde, Lectures on Amenability, Lectures Notes in Mathematics 774, Springer Verlag, 22. [] Francis Edward Su, The Banach-Tarski Paradox, Minor thesis, part of PhD, Harvard University, 99. [4] Robert M. French The Banach-Tarski Theorem 2, The Mathematical Intelligencer, vol., no. 4, Springer-Verlag New York, 988. The standard text on the subject. It contains (almost) everything you ever will want to know about the paradox. It also contains lot of open paradox-related problems. 2 A popular introduction to the ideas behind the proof of the theorem. This is where I found the notion of shifting towards infinity, as mentioned in the introduction. 5

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