An Exploration of the Cantor Set

Transcription

1 An Exploration of the Cantor Set Christopher Shaver, Rockhurst University 09 James and Elizabeth Monahan Summer Research Fellowship Summer 008 MT4960: Mathematics Seminar Spring 009 Abstract This paper is a summary of some interesting properties of the Cantor ternary set and a few investigations of other general Cantor sets. The ternary set is discussed in detail, followed by an explanation of three ways of creating general Cantor sets developed by the author. The focus is on the dimension of these sets, with a detailed explaination of Hausdor dimension included, and how they act as interesting examples of fractal sets. Keywords: Cantor set, fractal, Hausdor dimension, self-similarity

2 Introduction Georg Cantor (845-98) rst introduced the set that became known as the Cantor ternary set in the footnote to a statement saying that perfect sets do not need to be everywhere dense. This footnote gave an example of an in nte, perfect set that is not everywhere dense in any interval, a set he de ned as real numbers of the form x = c 3 + ::: + c v + ::: (.) 3v where c v is 0 or for each integer v.[] De nition (.): A set S is said to be perfect if S = S, where S is the set of all the limit points of S. In other words, S is perfect if S is a closed set in which every point is a limit point of S. De nition (.3): A set S is said to be nowhere dense if the interior of the closure of S is empty. The paper in which Cantor s statement and footnote were made was written in October of 88, but in a paper published in 875, H. J. S. Smith made the discovery of nowhere dense sets with positive outer content, meaning that they still take up space. This paper, in which Smith gave an example of a set that would today be classi ed as a type of Cantor set, went largely unnoticed until well after Cantor s discoveries were made. Now, in this paper, I attempt to study the Cantor ternary set from the perspective of fractals and Hausdor dimension and make my own investigations of other general Cantor sets I constructed and their dimensions. The term "Cantor set" is most often used to refer to what is known as the Cantor ternary set, which is constructed as follows: Let I be the interval [0; ]. Divide I into thirds. Remove the open set that makes up the middle third, that is, ( 3 ; 3 ), and let A be the remaining set. Then A = [0; 3 ] [ [ ; ]: (.4) 3

3 Continue by removing the open middle third segment from each of the two closed sets in A and call the remaining set A. So, A = [0; 9 ] [ [ 9 ; 3 ] [ [ 3 ; 7 9 ] [ [8 ; ]: (.5) 9 Continue in this fashion at each step k for k N, removing the open middle third segment from each of the closed sets in A k and calling the remaining set A k+. For each k N, A k is the union of k closed intervals each of length 3 k. De nition (.6): The Cantor ternary set, which we denote C 3, is the "limiting set" of this process, i.e. \ C 3 = A k. [] k= The Cantor ternary set is interesting in mathematics because of the apparent paradoxes of it. By the way it is constructed, an in nite number of intervals whose total length is are removed from an interval of length, so the set cannot contain any interval of non-zero length. Yet the set does contain an in nite number of points, and, in fact, it has the cardinality of the full interval [0; ]. So the Cantor set contains as many points as the set it is carved out of, but contains no intervals and is nowhere dense. We know that the set contains an in nite number of points because the endpoints of each closed interval will always remain in the set, but the Cantor set actually contains more than just the endpoints of the closed intervals A k. In fact, 3

4 4 C but is not an endpoint of any of the intervals in any of the sets A k. Notice rst that each element of C can be written in a ternary (base 3) expansion of only 0s and s. At every level of removal, every number with a ternary expansion involving a is removed. At the rst stage of removal, for instance, any number remaining would have the digit c = 0 or where x = 0:c c c 3 :::, since if x [0; 3 ], c = 0 and if x [ 3 ; ], c =. Repeating this argument for each level of removal, it can be shown that if x remains after removal n, c n is 0 or. Now, we can write 4 as 0:000::: in ternary expansion. At the kth stage of removal, any new endpoint has a form of either a in the 3 kth ternary place which repeats in nitely or terminates at the 3 (k ) ternary place. Since 4 = 0:000, it does not follow the pattern of the endpoints. Therefore, 4 is not an endpoint of the Cantor set, and there are in nitely many points like that. Properties of the Cantor set Certain easily proven properties of the Cantor ternary set, when they are pieced together, help to show the special nature of Cantor sets. The Cantor ternary set, and all general Cantor sets, have uncountably many elements, contain no intervals, and are compact, perfect, and nowhere dense. C 3 has uncountably many elements. We will show this by contradiction. Let C 3 = fx [0; ) : x has a ternary expansion involving only zeros and twosg. Suppose C 3 is countable. Then there exists f : N! onto C 3 by the de nition of countablility. Let X n = f(n) for all n N. So C 3 = fx ; x ; x 3 ; :::; x n ; :::g where: x = 0:c c c 3 ::: x = 0:c c c 3 :::. (.) x n = 0:c n c n c n3 ::: 4

5 . where c nm is either 0 or for all n; m. De ne c = 0:c c c 3 ::: by c = if = 0 c ; c = 0 if c = if = 0 c ; :::; c n = 0 if c = if = 0 cnn ; ::: (.) 0 if c nn = Clearly, c C 3. But, c 6= X n for any n, since c 6= x n in its 3 nth place. This is a contradiction. Therefore, C 3 is uncountable. C 3 contains no intervals. We will show that the length of the complement of the Cantor set C 3 is equal to, hence C 3 contains no intervals. At the k th stage, we are removing k intervals from the previous set of intervals, and each one has a length of 3 k. The length of the complement within [0; ] after an in nite number of removals is: X k ( 3 k ) = X ( 3 3 )k = X ( 3 3 )k = 3 k= k= k=0 = : (.3) 3 Thus, we are removing a length of from the unit interval [0; ] which has a length of. Therefore, the Cantor set must have a length of 0, which means it has no intervals. C 3 is compact. Using the Heine-Borel Theorem, which states that a subset of R is compact i it is closed and bounded, it can be shown rather easily that C 3 is compact. C 3 is the intersection of a collection of closed sets, so C 3 itself is closed. Since each A k is within the interval [0; ], C 3, as the intersection of the sets A k, is bounded. Hence, since C 3 is closed and bounded, C 3 is compact. So far we have that the Cantor set is a subset of the interval [0; ] that has uncountably many elements yet contains no intervals. It has the cardinality of the real numbers, yet it has zero length. C 3 is perfect. 5

6 By (.) a set is perfect if the set is closed and all the points of the set are limit points of the set. Since C 3 is compact, it is necessarily closed. For each endpoint in the set C 3 there will always exist another point in the set within a deleted neighborhood of some radius " > 0 on one side of that point since the remaining intervals at each step are being divided into in nitely small subintervals and since the real numbers are in nitely dense. Likewise, for each nonendpoint in the set there will always exist another point in the set within a deleted neighborhood of some radius " > 0 on both sides of that point. Hence, there must exist a deleted neighborhood of some radius " > 0 around each point of the set C 3 for which the intersection of that deleted neighborhood and the set is nonempty. Therefore, each point in the set is a limit point of the set, and since the set is closed, the set C 3 is perfect. C 3 is nowhere dense. By (.3) a set is nowhere dense if the interior of the closure of the set is empty. The closure of a set is the union of the set with the set of its limit points, so since every point in the set C 3 is a limit point of the set the closure of C 3 is simply the set itself. Now, the interior of the set must be empty since no two points in the set are adjacent to each other. At the in nite level of removal, if there did exist a series of adjacent points, that is an interval of points, the middle third section of that interval will be removed and the removal would continue on an in nitely small scale, ultimately removing anything between two points. Hence the set C 3 is nowhere dense. Dimension of the Cantor Set The Cantor set seems to be merely a collection of non-adjacent points, so should intuitively have a dimension of zero, as any random collection of non-adjacent points would have. In this sense, the topological dimension of the Cantor set is 0. But utilizing a di erent de nition of dimension, such as Hausdor dimension, allows us to see the fractional dimension of the Cantor set while still maintaining the integer dimensions of points, lines, and planes. 6

7 Hausdor Dimension De nition: Let E R n and let s and be positive real numbers. Let C (E) be the collection of all countable -covers of E, where a -cover of E is a sequence fu j g of subsets of R n whose union contains E and for which 0 < ju j j < for all j = ; ; :::. Then, X H s (E) := inff ju j j s : fu j g C (E)g (3.) j= [3]. De nition (3.): For any subset E of R n, H s (E) := sup H s(e).!0 De nition (3.3): The Hausdor s-dimensional measure is the restriction of H s () to the - eld of H s ()- measureable sets. The Hausdor dimension of E is de ned to be: dim(e) = inffs : H s (E) = 0g (3.4) [3]. In simple terms, Hausdor dimension involves rst considering the -covers of the set E. If you take the sum of the diameters of all -covers of E raised to the power of s, the in mum of that sum is H s (E). The Hausdor s-dimensional measure of E is then the sup H s (E). Finally, the Hausdor dimension of E, >0 denoted by dim(e), is the in mum of all the values of the real number s such that the Hausdor s-dimensional measure of E is 0. Hausdor dimension generalizes the concept of dimension of a vector space in such a way that points have Hausdor dimension 0, lines have Hausdor dimension, etc., but in general the Hausdor dimension of a set is not necessarily integer. Fractals are de ned as sets whose Hausdor dimension is greater than its topological dimension, with the Hausdor dimension of fractals speci cally non-integer. Theorem : The dimension of the Cantor ternary set (C k ) is: 7

8 log d = log( k ) (3.5) Proof: Let us create a series of -covers of C k : fu 0 = [0; ]g; a cover (3.6) fu = [0; fu = [0; 4 U 4 = [ k k ]; U = [ + k ; ]g; a ( k + 4k ]; U = [ 4 4k ; 4k ; ]g; a ( 4 k ) cover k ]; U 3 = [ + k ; k ]; k + ) cover () 4k In general, a ( k )g -cover is fu gj g g j= (3.7) For s > 0, X H s (C k ) : = inff ju i j s : fu i g C (C k )g (3.8) g X i= = g (( i= ju gi j s, for ( k )g ) s = k )g (( g k )g ) = s ( k )s g So, if ( k )s =, then H s (C k) for all > 0. Now, ( k )s = i log( k )s = log i s log( k ) = log. That is, s = log log( k log ). If s = log( ), then Hs (C k ) and k log dim(c k ) log( k ) (3.9) 8

9 Now we show H s (C k ), where s = log log( ). For > 0, let fv ig be a -cover of C k. We may assume k V i is open for each i. Since C k is compact, there is a nite subcover fv i g n i= ; V i = (a i ; b i ), where a i < a i+ and b i < b i+ for i n. Now, there exists a closed subinterval [ i ; i ] of (a i ; b i ) such that n[ C k [ i ; i ]; i < i < i+ i+ for i n ; (3.0) i= where i and i have the form m( k )g for all i. Let K = maxfk : i = m( k )g or i = m( k )g for some i; i n; ( ) doesn t divide mg k (3.) Then [ g U gj j= Let C g = g [ j= n[ i= [ i ; i ], where fu gj g g j= is as de ned in the rst part of the proof. U gj. Now consider [ i ; i ] nc g for some i, i n. [ i ; i ] nc g contains at least one open interval, I, of length at least ( )( i i ). Now [ i ; i ] ni is the disjoint union of two closed intervals, I k and I. By the concavity of f(t) = t s and since ( k )s = and ( i i ) ( k ) (ji j + ji j) we obtain: ( i i ) s [ ( k ) (ji j + ji j)] s (3.) = [ ji j + ji j] s ( ji j s + ji j s ) = ji j s + ji j s Reduce I and I similarly and obtain that ( i Consequently, X i ) s ju gi j s j:u gi[ i; i ] 9

10 nx jv i j s i= nx ( i i ) s (3.3) i= 0 X ju gi j s A i= j:u gj [ i; i ] = g X j= ju gj j s = g ( s k )g = ( k )s g = It follows that H s(c k) and hence H s (C k ). Therefore, dim(c k ) = log log( ). [3] k There is a shortcut for computing the Hausdor dimension of sets that are self-similar in nature. To be self-similar, the mappings of the set must be a nite collection of similitudes, that is, the mappings preserve the geometry of the set, such that the set is invariant with respect to the set of mappings, and there must exist a positive real number s such that H s of the set is positive but H s of the intersection of two di erent mappings of the set is zero. De nition (3.4): Let : R n! R n. If there exists a c, 0 < c <, such that j(x) (y)j c jx yj for any x and y in R n, then is called a contraction. De nition (3.5): Let be a contraction. Then inffc : j(x) (y)j c jx yj 8x; yr n g is called the ratio of the contraction. De nition (3.6): Let : R n! R n be a contraction and E be a subset of R n. If preserves the geometry of E ( is a combination of a translation, a rotation, a re ection, and/or a dilation) then is called a similitude. De nition (3.7): Let E R n and f i g k i= be a nite collection of contractions. k invariant with respect to f i g k i= if E = [ i (E). i= Then E is said to be De nition (3.8): Let E R n and f i g k i= be a nite collection of similitudes such that E is invariant with respect to f i g k i=. If there exists s > 0 such that Hs (E) > 0 but H s ( i (E) \ j (E)) = 0 for i 6= j, then E is called self-similar. 0

11 De nition (3.9): A nite collection of contractions f i g k i= is said to have the open set condition if there k[ exists a bounded open set W such that i (W ) W, and this union is disjoint. i= The Cantor set is self-similar by (3.8) since its mappings, all variations on 3x, preserve the geometry of the set and there is a positive Hausdor s-dimensional measure of the set but not of the intersection of two di erent mappings since no two mappings of the set have a non-empty intersection. The Hausdor dimension of a self-similar set can be found by using the following theorem: Theorem : Let f i g k i= be a collection of similitudes such that E Rn is invariant with respect to f i g k i=. If f ig k i= satis es the open set condition and r i is the ratio of the i-th similitude i, then the kx Hausdor dimension of E is equal to the unique positive numbers for which (r i ) s =. [3] The computation of the Hausdor dimension of the Cantor ternary set, C 3, follows very easily from Theorem. Proposition: The dimension of the Cantor ternary set C 3 is d = log log 3. i= Proof: Let (x) and (x) be de ned as: (x) = 3 x (x) = 3 x + 3 (3.0) [ Notice that C 3 = i (C 3 ). Also, f i g i= satis es the open set condition for W = (0; ). Applying i= the theorem with r = 3 and r = 3, we need to nd s such that X (r i ) s =. i= ( 3 )s = i s = log log 3 : (3.) [3]. dim(c 3 ) = log log 3.

12 General Cantor Sets Up to this point, our discussion of the Cantor set has been limited to what is known as the Cantor ternary set, de ned in the Introduction. We will now discuss some generalizations. My further investigations were motivated by a curiosity as to what would happen to the dimension of the set if the removal process was de ned di erently. I consider three di erent general methods of removal from the interval [0; ] that depend upon a natural number k. As will be shown, all three methods of removal are equivalent when k = 3, yielding the Cantor ternary set..) Method C: Let fc k g be the collection of sets de ned in terms of k, for k, in which each set in the sequence is formed by the repetitive removal of an open interval of length k from the center of each closed interval, starting with the interval [0; ]. In this way, the size of the closed intervals remaining on either side of the open interval removed will be ( k ). Since each of the sets C k are self-similar sets, we can use Theorem to nd the Hausdor dimension of each of the sets, which we will do in general for any k. Let (x) and (x) be de ned as: (x) = ( k )x (x) = ( k )x + + k (4.) Notice that C k = r = ( k [ i (C k ). Applying the theorem for self-similar sets with r = ( i= ), we need to nd s such that k ) and X (r i ) s =. (4.) i= So, ( k )s = i s = log( log (4.3) k ).

13 Thus, log dim(c k ) = log( k ). (4.4).) Method D: Let fd k g be the collection of sets de ned in terms of k, for k, in which each set in the sequence is formed by the repetitive removal of an open interval of length ( k ) from the center of each closed interval starting with [0; ], with intervals of length k remaining on each side. Notice that with this method of removal, we are varying the length of the side intervals in terms of k then removing the interval inbetween, whereas in the rst method of removal, method C, you are varying the length of the center interval in terms of k. Again, since each of the sets D k are self-similar sets, we can use Theorem to nd the Hausdor dimension of each of the sets, which we will do in general for any k. Let (x) and (x) be de ned as: (x) = k x (x) = k x + k (4.5) [ Notice that D k = i (D k ). i= need to nd s such that Applying the theorem for self-similar sets with r = k and r = k, we X (r i ) s =. (4.) i= So, ( k )s = i s = log log k. (4.6) 3

14 Thus, dim(d k ) = log log k. (4.7) 3.) Method E: Let fe k g be the collection of sets de ned in terms of k, for k, in which each set in the sequence is formed by the repetitive removal of alternating open intervals of length k from each closed interval, starting with [0; ], when each closed interval is divided into k subintervals. This method of removal leads to two similar but distinctly di erent cases. When k is odd, k alternating sections are removed from each closed interval, leaving each interval of length k. When k is even, ( k ) alternating sections are removed from each closed interval, leaving one interval of length k on the left end and two full intervals each of length k adjacent to each other on the right end. Since di erent mappings will be required in order to generate the sets, each of the two cases yields sets with di erent Hausdor dimensions. In the case where k is odd, using Theorem to nd the Hausdor dimension of these self-similar sets, we let (x), (x),..., k+ (x) be de ned as: (x) = k x (x) = k x + k (4.8) ; :::; k+ (x) = k x + k k Note that the number of i mappings needed is determined by the value of the natural number k, with k+ mappings needed when k is odd. Applying the theorem for self-similar sets with r = k, r = k,..., r k+ = k, we need to nd s such that X (r i ) s =. (4.) k+ i= So, 4

15 ( k + )( k+ log k )s = i s = log k. (4.9) Thus, when k is odd, dim(e k ) = log k+ log k. (4.0) In the case where k is even, we let (x), (x),..., k+ (x) be de ned as: (x) = k x (x) = k x + k (4.) ; :::; k (x) = k x + k k for = ; ; :::; ( k ). Note that the number of i mappings is determined by the natural number k, with k mappings needed when k is even. Applying the theorem for self-similar sets with r = k, r = k, r k = k, we need to nd s such that k X (r i ) s =. (4.) i= So, ( k )( k )s + ( k )s =. (4.) Simplifying this equation to ( k ) = k s s, it is not noticeably solvable directly for a general k, but the dimension of each set is equal to the value of s that satis es the equation for its given k. 5

16 The di erences between the sets formed under each method of removal are apparent when k = 4, and the di erences between di erent values of the natural number k for a given method of removal are apparent by comparing k = 4 with k = 5. Set C 4 : Set C 5 : Set D 4 : Set D 5 : Set E 4 : 6

17 Set E 5 : Note that when k =, the amount being removed under methods D and E is equal to 0, and the dimension is consequently equal to in all cases, since we are left with the full interval [0; ]. Note: C 3 = D 3 = E 3. Proof: The set C 3 is formed by the repetitive removal of an open interval of length 3 from the center of each closed interval, starting with the interval [0; ] which leaves closed intervals of size 3 on each side. Since the set D 3 is formed by the repetitive removal of an open interval of length ( 3 ) = 3 from the center of each closed interval starting with [0; ], with intervals of length 3 remaining on each side, it is equivalent to the set C 3. Also, since the set E 3 is formed by dividing the interval [0; ] into 3 subintervals and removing alternating sections, which is only the center section, with intervals of length 3 on each end, it is equivalent to both C 3 and D 3. Hence, C 3 = D 3 = E 3. Also note that the calculations of dimension for each set yield the same dimension, which must be the case since the three sets are the same. So, log dim(c 3 ) = log( 6 ) = log log 3. dim(d 3) = log 3+ log 3. dim(e log 3) = log 3 = log log 3 (4.3) Thus, dim(c 3 ) = dim(d 3 ) = dim(e 3 ). In each method of removal, what happens to the dimension as k approaches in nity? In method C, dim(c k ) = log log( ). k So, lim dim(c k) = lim k! k! log( log k ) = log log =. (4.4) Hence, when [0; ] is divided into 3 subintervals, the smaller the length of the interval k closer the Hausdor dimension gets to. removed, the 7

18 In method D, dim(d k ) = log log k. So, log lim dim(d k) = lim = 0. (4.5) k! k! log k Hence, when [0; ] is divided into 3 subintervals, the smaller the length of the side intervals k, that is, the larger the length of the interval ( k ) removed, the closer the Hausdor dimension gets to 0. In method E, dim(e k ) = log k+ log k when k is odd. So, lim dim(e k) = lim k! k!! log k+ =. (4.6) log k Hence, when [0; ] is divided into an odd number of equal length intervals, the larger the number of such intervals, the closer the Hausdor dimension gets to. Conclusion The Cantor ternary set and the general Cantor sets are all examples of fractal sets. Their self-similarity allows their Hausdor dimention to be calculated easily, and each is shown to be non-integer. Many questions for further investigation remain, including expoloring other methods of removal. The ultimate question is whether or not it is possible to nd a method of removal that will yield a speci c given Hausdor dimension. Considerable work and exploration would need to be done in order to determine this. Acknowledgements This paper is the product of a research project undertaken in Summer 008 through the Department of Mathematics and Physics at Rockhurst University in Kansas City, Missouri with Dr. Zdeñka Guadarrama, Assistant Professor, and Jeanette Powers, undergraduate student. The project was extended with further research for the MT4960 Mathematics Seminar course in Spring 009 at Rockhurst. The author and the project were generously supported through the James and Elizabeth Monahan Summer Research Fellowship. The author wishes to thank Mr. and Mrs. Monahan and especially Dr. Guadarrama and Ms. Powers. 8

19 References. J. Fleron, "A Note on the History of the Cantor Set and Cantor Functions", Mathematics Magazine. 67 (994) S. Lay, Analysis, With an Introduction to Proof, Fourth Edition, Prentice Hall, Upper Saddle River, NJ, K.J. Falconer, The Geometry of Fractal Sets, Cambridge University Press, Cambridge,