THE 2-ADIC, BINARY AND DECIMAL PERIODS OF 1/3 k APPROACH FULL COMPLEXITY FOR INCREASING k

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1 #A28 INTEGERS 12 (2012) THE 2-ADIC BINARY AND DECIMAL PERIODS OF 1/ k APPROACH FULL COMPLEXITY FOR INCREASING k Josefina López Villa Aecia Sud Cinti Chuquisaca Bolivia Peter Stoll Villa Aecia Sud Cinti Chuquisaca Bolivia Received: 2/28/11 Revised: 12/10/11 Accepted: /2/12 Pulished: 4/18/12 Astract An infinite word x over an alphaet with letters has full complexity if for each m N all the m words of length m are factors of x We prove that the periods of ±1/ k in the 2-adic expansion approach full complexity for increasing k: For any m N the periods for k > (m + 1)ln(2)/ln() have complexity 2 m Amazingly these 2 m words occur in the period almost the same numer of times On the way first we prove the same for the inary period We get a similar result for the decimal period of 1/ k 1 Introduction Let Z 2 denote the ring of 2-adic integers Each x Z 2 can e expressed uniquely as an infinite string x 0 x 1 x 2 of 1 s and 0 s The x k are the digits of x written from left to right For instance 1 = 1111 and 1 = 1000 since = 0 The 2-adic norm 2 in Z 2 is given y x 2 := 2 n if x 0 where x n is the first nonzero digit of x and x 2 := 0 if x = 0 Let 0 d 0 < d 1 < d 2 < e a finite or infinite sequence of nonnegative integers defined y d i := k whenever x k = 1 for a 2-adic integer x = x 0 x 1 x k Then x can e written as the finite or infinite sum x = 2 d0 + 2 d1 + 2 d2 + The x + 1 map T is defined on the 2-adic integers Z 2 y T (x) = x/2 for even x and T (x) = (x + 1)/2 for odd x Let T k (x) denote the k-th iterate of T and T 0 (x) := x The sequence (T k (x) mod 2) k=0 is called the parity vector of x Z 2 (Lagarias []) The parity vector can e regarded

2 INTEGERS: 12 (2012) 2 as the 2-adic integer v x = (T k (x) mod 2) 2 k k=0 Conversely each x Z 2 is uniquely determined y its parity vector v x Indeed there is an explicit formula (Bernstein [1]): x = Φ(v x ) where Φ(2 d0 + 2 d1 + 2 d2 + ) := i 0 1 i+1 2di (1) The periodicity conjecture of the famous x + 1 prolem states that the parity vector v x is eventually periodic for all x Z 2 (Bernstein Lagarias [2]) This conjecture is still open If the parity vector of some x Z 2 is eventually periodic then x = Φ(v x ) is also eventually periodic This follows from (1) y computing the corresponding geometric series using the 2-adic norm For instance 1/ = since = 1/(1 4) If the periodicity conjecture is true Φ maps each aperiodic v x onto an aperiodic 2-adic integer x Z 2 In a former paper ([5]) we calculated Φ(v x ) over a Sturmian word (the digits of v x are Sturmian) These aperiodic words are very similar to the periodic words y their almost-periodicity We accumulated evidence that Φ(v x ) is an infinite word of full complexity That was reason enough for searching more in this direction George Pólya in his famous ook How to solve it suggested that When you can t solve a prolem and we cannot prove the periodicity conjecture then there is an easier prolem you can solve: find it That is what we did We prove that the terms 2 di / i+1 added in (1) approach full complexity for increasing i The complexity function of a word x counts for each nonnegative integer m 0 the numer P (x m) of different factors of length m in x 1 The numer P (x 1) counts the different letters appearing in x In our case the alphaet is {0 1} so P (x 1) = 2 If x is purely periodic with a period of length m we have P (x m) = m If x is Sturmian we have P (x m) = m+1 for every m 0 (Lothaire [6]) An infinite word x over {0 1} has full complexity if for every nonnegative integer m all the 2 m words of length m are factors of x We prove that for every m N the period p of 1/ i+1 has complexity P (x m) = 2 m for all (i + 1) > (m + 1)ln(2)/ln() The d i s in (1) introduce d i leading 0 s for each 1/ i+1 In this sense our est result is Corollary 4 which gives the necessary and sufficient condition for complexity 2 m Luckily there is a helpful relation etween the 2-adic and the inary expansion for reduced fractions with odd denominator of the unit interval This relation is given y Lemma 19 in Section 4: The 2-adic period of 1/ i+1 is the reversal of the inary period of +1/ i+1 Therefore the main part of this paper is in Section where we study the inary expansion of 1/ k The transcription from inary to 2-adic periods is in Section 4 and our main result appears there as Corollary 4 of 1 Lothaire [6] A word f is called a factor of a word x if there exist words u v such that x = ufv

3 INTEGERS: 12 (2012) Theorem 1 Most things we deal with in Section are well known facts of elementary arithmetic and the recursive method used there can e generalized to other ases and also to other fractions 1/p in ase 10 for primes p different from 2 and 5 As such an application should e seen Section 5 where we study the decimal expansion of 1/ k The main tool for estalishing conditions for complexity is Proposition 2 in Section 6 2 Results Theorem 1 Let k N and m N \ {1} The set X m (k) of words with length m in the inary expansion of 1/ k has 2 m elements if and only if k k 0 where k 0 = (m + 1) ln(2) ln() All these 2 m words can e found in the enlarged period p k := p k (0) m 1 which is the period p k of 1/ k concatenated with the first m 1 zeros of p k The words n X m (k) k k 0 occur in p k with almost the same frequency f(n): 2 f(n) f(n ) 2 for all n n X m (k) Theorem 2 Let k k 0 and m e as in Theorem 1 If k > k 0 then the inary period p k of 1/ k has complexity 2 m Theorem The inary period of 1/ k has the following properties: p k = x 1 x 2 x i x lk (l k = 2 k 1 ) 1 The digits are given y x i = (2 i mod k ) mod 2 or equivalently the digit x i is 1 if and only if the remainder 2 i mod k is odd 2 The numer n 0 of 1 s at an even position exceeds the numer n 1 of 1 s at an odd position y 1: n 0 n 1 = 1 The word p k has the same numer of 1 s and 0 s 4 The right half of p k is complementary to the left half of p k : x i+lk /2 + x i = 1 (0 i l k /2) 5 In the left half of p k the numer of 1 s at an even position is equal to the numer of 1 s at an odd position 2 The frequency f(n) is the numer of times a given word n occurs as factor of the enlarged period

4 INTEGERS: 12 (2012) 4 6 There are exactly k ln()/ln(2) leading 0 s in p k Corollary 4 Let k N and m N \ {1} The set X m (k) of words with length m in the 2-adic expansion of 1/ k has 2 m elements if and only if k k 0 where k 0 = (m + 1) ln(2) ln() All these 2 m words can e found in p k := (0) m 1 p k which is the period p k of 1/ k extended to the left y m 1 zeros The words n X m (k) k k 0 occur in p k with almost the same frequency f(n): f(n) f(n ) 2 for all n n X m (k) Corollary 5 Let k k 0 and m e as in Corollary 4 If k > k 0 then the 2-adic period p k of 1/ k has complexity 2 m Corollary 6 Let k k 0 and m e as in Corollary 4 Let p + k e the period of the (not purely periodic) 2-adic expansion 1/ k = 1p + k p+ k If k > k 0 then the 2-adic period p + k has complexity 2m Theorem 7 Let k N and m N The set X m (k) of words with length m in the decimal expansion of 1/ k has 10 m elements if and only if k k 0 where k 0 = m ln(10) + 2 ln() All these 10 m locks of m digits can e found in the enlarged period u k := u k (0) m 1 which is the period u k of 1/ k concatenated with the first m 1 zeros of u k The words n X m (k) k k 0 occur in u k with almost the same frequency f(n): f(n) f(n ) 1 for all n n X m (k) Corollary 8 Let k k 0 and m e as in Theorem 7 If k > k 0 then the decimal period u k of 1/ k has complexity 10 m The Binary Expansion In this section we use the standard notation for numers in ase 2 with the most significant digit at the left For instance 1101 = = 1 and = = 1/16 The inary expansion of 1/ k is a purely periodic fraction The period is a word p k = x 1 x 2 x i x lk (l k = 2 k 1 ) If m = 1 then (0) m 1 := ɛ (the empty word)

5 INTEGERS: 12 (2012) 5 of digits 0 and 1 with length l k = 2 k 1 : 1 k = x i 2 i = 2lk i=1 2 l k 1 lk i=1 x i 1 2 i (2) For example p 1 = 01; p 2 = ; p = ; p 4 = We get the period p k+1 dividing 1/ k y executing a long division y the inary integer 11 As l k+1 = l k we write the word p k three times uilding the word p k p k p k as dividend to otain the full period p k+1 The long division in short form looks like this: y 1 y i y lk y 1 y i y l k y 1 y i y l k 11 ) x 1 x i x lk x 1 x i x l k x 1 x i x l k r 1 r i r lk r 1 r i r l k r 1 r i r l k The divisor 11 and the dividend p k p k p k are in the second line The quotient is in the first line where the 0 has een omitted At each step after ringing down the next x i we have a remainder r i These remainders are in the third line We define r 0 := 0 at the start of the division Note that x i = x i = x i for each i L k where L k := {1 2 l k } There are only three possile remainders less than 11: {0 1 10} Therefore {r i r i r i } {0 1 10} Oviously r It is a simple ut important fact that the remainders r i r i of (0 1 10) l k = 0 Since l k+1 = l k r lk 0 and r l k 0 are a permutation and r i Lemma 9 Let L k := {1 2 l k } and i L k The sequence (r i r i r i ) is a permutation of (0 1 10) Proof (a) The Lemma holds for i = l k When ringing down the first digit x 1 of p k we take care of the previous remainder r lk 0 If r lk = 1 the division continues y dividing the inary integer 1x 1 = x y 11 If r lk = 10 then we continue with 10x 1 = x The division can e done y ringing down a single digit x 1 ut also y ringing down two or more digits in one step When we ring down all the digits of p k in one step the division y 11 continues with the inary integers 1x 1x 2 x l k = 1 2 l k + x 1 x 2 x lk for r lk = 1 or 10x 1x 2 x l k = 1 2 l k l k + x 1 x 2 x lk for r lk = 10

6 INTEGERS: 12 (2012) 6 since p k = p k The new remainder is r l k in oth cases As l k is even we have 2 l k 1 (mod ) so that r l k 1 + r lk (mod ) for r lk = 1 and r l k 2 + r lk (mod ) for r lk = 10 Hence (r lk r l k r l k ) is either (1 10 0) or (10 1 0) () The Lemma holds for i < l k We have x 1 x 2 x i = x 1 x lk x 1 x i The remainder of the left side is r i The remainder of the right side is equal to the remainder of r lk x 1 x i where r lk is either 1 or 10 Dividing the inary integer r lk x 1 x i y we get the remainder r lk 2 i + r i modulo Hence r i r l k 2 i + r i (mod ) In the same way x 1 x i = x 1 x l k x 1 x i yields r i r l k 2 i + r i (mod ) There are 12 possiilities: r lk r l k r i r i r i i even i odd In all cases (r i r i r i ) is a permutation of (0 1 10) A word x of length m entirely contained in p k has two exact copies (x and x ) in the dividend p k p k p The input remainders k : x is factor of p k and x is factor of p k and r i 1 and also the output remainders r i+m 1 r i+m 1 r i 1 r i 1 and r i+m 1 are all different y Lemma 9 so that the corresponding words y y and y in the quotient are all different too It is convenient to use ase 10 numers Each word x of length m receives a list numer n from 0 to 2 m 1 which is its value when read as a ase 2 integer For instance if m = the list numer 6 is the word x = 110 When translating a list numer ack into a word we take care of eventually leading 0 s For instance if m = 4 the list numer 5 is the word x = 0101 Dividing n y we get the quotient q which is the list numer of the word y The possile remainders are 01 and 2 We identify the words x or y with their respective list numers n or q for a given m N \ {1} We denote the set of list numers y M inary := {0 1 2 m 1} () Tale 1 shows the division y for any m N \ {1} The construction of Tale 1 is quite easy Write the list numers from 0 to 2 m 1 in the first column Write

7 INTEGERS: 12 (2012) 7 each of them three times consecutively in column 2 starting with (000) When column 2 is filled up continue with the remaining numers in column 4 and finally in column 6 Write the remainders (012) in this order filling up columns 5 and 7 r i 1 = 0 r i 1 = 1 r i 1 = 2 n q r i+m 1 q r i+m 1 q r i+m m 2 2m m mod (m + 1) mod m 1+2 2m n n n mod n+2m (n + 2 m ) mod n+2 2m (n m ) mod 2 m 1 2m 1 m mod 2 2 2m 1 (m + 1) mod 2 2m 1 2 Tale 1: M inary quotients and remainders Definition 10 Let m N \ {1} The set of triads for m is defined y { ( n n + 2 m n m ) } S inary := n M inary We exclude m = 1 ecause q {0 1} would generate two times the same numer in the triad For m > 1 the numers of the triad are all different Definition 11 Let m N \ {1} Let p = p k p k = x 1 x 2 e the infinite word of the inary expansion of 1/ k The enlarged period of 1/ k is the word p k = p k x 1 x 2 x m 1 We denote y X m (k) the set of (different) words of length m appearing in p In the context it will e clear if its elements are words or list numers identifying them Lemma 12 For each x X m (k) the word x is factor of p k Proof Let x = x i x i+m 1 X m (k) Take j := i mod l k If j 0 then x j x j+m 1 is factor of p k If j = 0 then i is the last digit in some p k and the word x lk x 1 x 2 x m 1 is factor of p k

8 INTEGERS: 12 (2012) 8 Let p k e the enlarged period of 1/ k for some m We determine the enlarged period p k+1 y executing the long division 11)p k p k p k = p k+1 We start with an example In Example 1 we divide the enlarged period p 2 of 1/9 y to get the enlarged period p of 1/27 We do this for m equal to 2 and 4 In the three cases the dividend elow the division line is p 2 p 2 p 2 and the quotient aove the division line is p The smaller numers are list numers which can e read in dividend and quotient y grouping the digits in locks of 2 or 4 digits corresponding to m Example 1 Consider For instance if m = we can read the following words in the dividend from left to right: The corresponding list numers are In the quotient we can read the list numers For m = the 7 in the dividend generates 2 7 and 5 in the quotient Indeed the list numer 7 generates the triad (257) in Tale 1 Note that these numers appear permuted In Example 1 we have X 2 () = {0 1 2 } = X 2 (2) Further X () = { } and X (2) = { } Finally X 4 () = { } and X 4 (2) = { } If m > 6 the words in the dividend (and also in the quotient) will e overlapping ecause the length of p 2 is 6 The example shows that X m (k) determines X m (k + 1) y means of tale 1 Proposition 14 We have { 2 m 2 2 m } X m (1) = where m N \ {1} Proof We have p 1 = 01 Let p := p 1 p 1 = x 1 x 2 Thus p 1 = p 1 x 1 x 2 x m 1

9 INTEGERS: 12 (2012) 9 There are only two words of length m: n (1) := 01 and n (2) := 10 oth of length m Word n (1) If m is odd then the last digit of n (1) is 0 Hence n (1) = m 2 We get n (1) = (2 m 2)/ So n (1) = 2 m / since 2 m 2 (mod ) If m is even then the last digit of n (1) is 1 Hence n (1) = m 2 We get n (1) = (2 m 1)/ = 2 m / since 2 m 1 (mod ) Word n (2) For odd m n (2) = m 1 = (2 m+1 1)/ = 2 m+1 / For even m n (2) = m 1 = (2 m+1 2)/ = 2 m+1 / Proposition 15 We have X m (k + 1) = { n n + 2 m n m } where k N m N \ {1} n X m(k) Proof We execute the long division 11)p k p k p k = p k+1 The words are identified y their list numers in M inary We use the notation n (i) and q (i) for words of length m having their first inary digit at position i The set of all words of length m in p k p k p k is given y X m (k) = {n (i) i L k } We have n (i) = n (i+l k) = n (i+2l k) Their input remainders r i 1 r i+lk 1 and r i+2lk 1 and also their output remainders r i+m 1 r i+lk +m 1 and r i+2lk +m 1 are all different y Lemma 9 It follows that the corresponding quotients q (i) q (i+l k) and q (i+2l k) are all different too Conversely if s (j) is a factor of length m in p k+1 for some j {1 2 l k } then s (j) {s (j mod l k) s (j mod l k+l k ) s (j mod l k+2l k ) } These factors are generated y n (j mod l k) = n (j mod l k+l k ) = n (j mod l k+2l k ) All these n s are equal to n (j) So X m (k + 1) = i L k {q (i) q (i+lk) q (i+2lk) } The q s are a permutation of ( n (i) / (n (i) + 2 m )/ (n (i) m )/) (Tale 1) Proposition 16 Let k N and m N \ {1} X m (k) = { t 2m k t 0 (mod ) and 0 < t < k } Proof Proposition 14 and Proposition 15 define a tree of triads Note that the two numers of X m (1) are elements of the triad generated y 0 in S inary Any n M inary generates the following tree of triads (we show only the levels k = 1 and k = 2): ( n n + 2 m n m ) { }} {{ }} {{ }} { n n + 2 m n m n m n m n m n m n m n m

10 INTEGERS: 12 (2012) 10 The k numers written at level k have the form (n + t 2 m )/ k where t {0 1 2 k 1}; eventually not all of them are different The left ranch of the tree corresponds to t 0 (mod ) the central ranch to t 1 (mod ) and the right ranch to t 2 (mod ) We sustitute n = 0 and cut off the left ranch All the remaining numers at level k are elements of X m (k) y Proposition 14 and Proposition 15 Note that card(x m (k)) 2 k Lemma 17 There is a unique t 0 2 (mod ) such that t 0 2m k t 0 2m k { 1 k 2 k } Proof The congruence t 2 m (mod k ) has the unique solution t 1/2 m (mod k ) and t 2 m (mod k ) has the unique solution t 2/2 m (mod k ) Hence t 2 m / k + 1/ k = t 2 m / k and t 2 m / k + 2/ k = t 2 m / k where t and t are taken as positive integers less than k Both are positive integers relatively prime to k Either t 1 (mod ) and t 2 (mod ) or t 2 (mod ) and t 1 (mod ) The claimed t 0 is either t or t Proposition 18 Let k N and m N \ {1} X m (k) = M inary if and only if 0 < 2m k < 1 2 Proof Let t 0 2 (mod ) e as in Lemma 17 Then t (mod ) and t is left out in X m (k) y Proposition 16 By Proposition 2 paragraph 1 { t2 m / k t {0 1 k 1}} = M inary if and only if 0 < 2 m / k < 1 As shown in the following figure we can avoid that the integer s := (t 0 + 1)2 m / k is left out in X m (k) y the condition 2 2 m / k < 1 + c/ k for the corresponding c {1 2} of Lemma 17 having now s = (t 0 + 2)2 m / k t 0 s t t }{{}}{{} c/ k 1 By Lemma 22 there is no other t 2 (mod ) such that t2 m / k is closer to (t2 m / k than for t 0 It follows that the condition 2 m+1 < k + c is necessary and sufficient to guarantee X m (k) = M inary Recall that m 2 and k 1 Oviously 2 m+1 = k is impossile We show that 2 m+1 = k +1 is impossile too Note that in this case m+1 is even since 2 m+1 1 (mod ) We get (2 (m+1)/2 1)(2 (m+1)/2 + 1) = k Only one of the consecutive integers (2 (m+1)/2 1) 2 (m+1)/2 and (2 (m+1)/2 + 1) is divisile y so either 2 (m+1)/2 1 = 1 and 2 (m+1)/2 + 1 = k or 2 (m+1)/2 1 = k and 2 (m+1)/2 + 1 = 1 We get the excluded value m = 1 (and k = 1) Hence 2 m+1 < k

11 INTEGERS: 12 (2012) 11 Proof of Theorem 1 By Proposition 18 X m (k) = M inary if and only if 0 < 2 m / k < 1/2 Thus k > (m + 1) ln(2)/ln() Hence k 0 = (m + 1) ln(2)/ln() The same condition yields m < k ln()/ln(2) 1 so m k ln()/ln(2) 1 The numer z of leading zeros in (2) is given y z = max{i 1/2 i > 1/ k } so z = k ln()/ln(2) Thus m 1 < z and p k = p k (0) m 1 All the 2 m words can e found in p k y Lemma 12 It remains to prove the last part of Theorem 1 We have k k 0 so X m (k) = M inary There is a q N such that k = q 2 m + r where 0 < r < 2 m Then q = k /2 m By Proposition 2 paragraph there are r different numers in M inary each of them occurring exactly q + 1 times in S 0 = ( t2 m / k ) k 1 t=0 ; each of the remaining 2 m r numers of M inary occurs exactly q times in S 0 Clearly 0 f(n) f(n ) 1 for all n n M inary By Propositions 16 and 18 when leaving out each third term of S 0 we get a sequence S 0 which also contains all the elements of X m (k) In any sequence with s elements y leaving out each third term we eliminate s/ or s/ elements as suggested in the following scheme (the crosses signify eliminated ): s: x x x x x x x x x x x Each numer n M inary repeats in S 0 either q or q + 1 times In S0 we have eliminated q/ or q/ elements for some n M inary occurring q times and (q + 1)/ or (q + 1)/ elements for another n M inary occurring q + 1 times We get { q q f(n) q q 1 and } f(n ) { q + 1 q + 1 q q + 1 } It is an easy check that in the four possile cases holds f(n) f(n ) 2 Proof of Theorem 2 Let p k0 p k0 p k0 = p k0 p k0 (p k0 t) e the dividend and p k0+1t the quotient where t is the tail of m 1 zeros We have X m (k 0 ) = M inary By Proposition 15 X m (k 0 + 1) = n X { n n+2m m(k 0) n+2 2m } Every n M inary generates a triad Conversely the numer a (respectively or c) of a triad (a c) is generated y three consecutive list numers as suggested in Tale 2 We define T := {n M inary the last inary digit of n is in t} and T := {q M inary the last inary digit of q is in t} for the corresponding quotient Every q T is generated y three consecutive list numers Since at least one of three

12 INTEGERS: 12 (2012) 12 n-2 a c n-1 a a c c n a c or a c or a c n+1 c a a c n+2 c a Tale 2: Three consecutive list numers (m odd) consecutive list numers say z must have its last inary digit equal to 1 the numer q is generated y z T so that we do not need T anymore Proof of Theorem 1 Let 2 i = q i k + r i where 0 < r i < k and i N Hence q i r i (mod 2) Note that r i = 2 i mod k We prove y induction the following statement: r i x i (mod 2) and q i = x 1 x 2 x i (written as a inary integer) The statement holds for i = 1 Recall that k 1 We have 2 = q 1 k +r 1 where 0 < r 1 < k So r 1 = 2 Further x 1 = 0 since 1/ k < 1/2 Hence r 1 x 1 (mod 2) and q 1 = x 1 since q 1 = 0 Assume the statement holds for some i Let 2 i+1 = q i+1 k + r i+1 where 0 < r i+1 < k So 2 i+1 = 2q i k + 2r i If 2r i < k then r i+1 = 2r i so r i+1 0 (mod 2) Further q i+1 = 2q i = x 1 x i 0 where x i+1 = 0 If 2r i > k then r i+1 = 2r i k so r i+1 1 (mod 2) Further q i+1 = 2q i + 1 = x 1 x i 1 where x i+1 = 1 In oth cases r i+1 x i+1 (mod 2) and q i+1 = x 1 x i+1 2 Write the remainders 2 i mod k as a sequence R of increasing natural numers: R := ( s 2 + s k 2 k 1) 1 s: s even s odd 1 In the second line are written the 1 s corresponding to the odd remainders y property 1 If r = 1+s is odd then i is odd too since 2 i 1+s (mod k ) implies 2 i 1 (mod ) If r = 2 + s is odd then i is even since 2 i 2 + s (mod k ) implies 2 i 2 (mod ) Therefore the odd remainders (1 5 7 k 2) have the alternating positions (even odd even odd even) The last remainder in this sequence corresponds to an even position i since there are l k /2 = k 1 odd remainders and k 1 /2 pairs (even odd) Hence n 0 n 1 = 1 There are l k /2 = k 1 odd remainders each of them producing a 1 The remaining k 1 remainders produce a 0 4 We have 2 l k 1 (mod k ) So 2 l k/2 1 (mod k ) since l k is even Thus 2 l k/2+1 1 (mod k ) Hence

13 INTEGERS: 12 (2012) (mod k ) and 2 l k /2+1 1 (mod k ); (mod k ) and 2 l k /2+2 1 (mod k ); 2 l k /2 1 (mod k ) and 2 l k 1 (mod k ) Then 2 i + 2 l k/2+i 0 (mod k ) for all i (1 2 l k /2) It follows r i + r lk /2+i = k As k is odd one of the two remainders is odd and the other one is even By property 1 x i + x lk /2+i = 1 5 The word w := (0) l k/2 (1) l k/2 has the properties 2 and 4 of Theorem We start with w and try to construct p k moving digits from one side to the other without violating the properties 2 and 4 The positions i in the left half and the corresponding positions l k /2 + i in the right half of w have different parity We cannot exchange some 1 with any 0 ecause the property 4 remains valid if and only if 1 and 0 are placed at corresponding positions l k /2+i and i When we exchange a 1 with the corresponding 0 at the left side either n 0 or n 1 is decreasing 1 initially we had n 0 n 1 = 1 so either n 0 1 (n 1 + 1) = 1 or n (n 1 1) = y property Therefore we have to exchange the digits y pairs so that the numer of 1 s in the left half must e even When we move a pair of 1 s oth having an even position n 0 n 1 will e altered: n 0 increases 2 and n 1 decreases 2 so n 0 n 1 = 5 If oth have initially an odd position n 0 decreases 2 and n 1 increases 2 so n 0 n 1 = When we move two 1 s to the left side their starting positions must have different parity so that n 0 n 1 = 1 is preserved 6 By property 1 max{i 2 i < k } = k ln()/ln(2) 4 The 2-Adic Expansion Lemma 19 Let p k = x 1 x 2 x lk e the period of the inary expansion of 1/ k (k N) The 2-adic expansion of 1/ k is purely periodic too and its period p k is the reversal of p k : p k = x lk x 2 x 1 Proof The ring Z 2 of 2-adic integers has no zero divisors so there is the field of fractions Q 2 of 2-adic numers Every non-integer 2-adic numer can e written as i= s x i 2 i where s > 0 is some natural numer The inary expansion (2) can e written as 1 k = 1 2 l k 1 2 l k l k i=1 x i 1 2 i We evaluate this expression term y term in the field Q 2 of 2-adic numers We get the 2-adic integer 1 2 l k 1 = 1 00 }{{ 0 } 1 00 }{{ 0 } 2 adic l k 1 zeros

14 INTEGERS: 12 (2012) 14 since 1 = l k 1 = 11 1 }{{} l k ones 11 1 }{{} l k ones and 1 00 }{{ 0 } l k 1 zeros }{{} = 111 }{{ } Further we get the 2-adic non-integer l k 1 1 x i 2 i = x i 2 i = x i 2 i + 0 i=1 2 adic i= l k i= l k where x i := x i for l k i 1 and x i = 0 for i 0 Note that the word x lk x 2 x 1 is the reversal of p k We multiply the last sum y 2 l k to get the 2-adic integer p k 00 where p k = x lk x 2 x 1 Finally }{{ 0 } 1 00 }{{ 0 } p k00 = p k p k l k 1 zeros Proof of Corollary 4 By Lemma 19 the word (0) m 1 p k is the reversal of the enlarged period p k (0) m 1 If one of the two words has complexity 2 m then also the other one so that the necessary and sufficient condition of Theorem 1 holds here too The frequency of non-palindromic words will possily change ut the difference of frequencies will not Proof of Corollary 5 Apply Lemma 19 Proof of Corollary 6 We define the rotate left operator ρ y ρ(x 1 x 2 x x lk ) := x 2 x x lk x 1 Then 1 k 2 adic = p k p k = 1ρ( p k )ρ( p k ) We need the additive inverse +1/ k 2 adic Such a change of sign can e done in Z 2 y exchanging 1 s and 0 s with exception of the first 1 For instance 1/ = and 1/ = So 1 k 2 adic = 1p + k p+ k where p + k denotes ρ( p k) with its 1 s and 0 s exchanged: p + k + ρ( p k) = 11 }{{ 1 } l k ones The rotate left operator ρ preserves the set of words of length m y moving the first word of p k to the end of ρ( p k ); it also preserves the complementarity of left and right halves Hence if p k has complexity 2 m then ρ( p k ) has complexity 2 m too Since all 2 m words of length m are factors of ρ( p k ) p + k contains the same words We can even say where they can e found: we get p + k = ρlk/2 ( p k ) y iterating ρ on p k exactly l k /2 times

15 INTEGERS: 12 (2012) 15 5 The Decimal Expansion The decimal expansion of 1/ k is purely periodic Let l k denote the length of the period u k Then l 1 = 1 and l k = k 2 for k 2 so l k is now odd The words are over the alphaet { } Furthermore 10 i 1 (mod ) for all positive integers i Since the long division in ase 10 is essentially the same algorithm as in ase 2 we have the following Lemma similar to Lemma 9 Lemma 20 Let k N \ {1} and i {1 2 k 2 } The sequence (r i r i r i ) is a permutation of (0 1 2) Since 10 i 1 (mod ) the proof is simpler and leads to only six possiilities (corresponding to the six permutations of (0 1 2)) equivalent to the case i even in ase 2 We define for m N: M decimal := { m 1}; (4) { ( n n + 10 m n m ) } S decimal := n M decimal (5) There is no special case m = 1 in ase 10 The numers of any triad of S decimal are all different even in the case m = 1 Definition 11 is still valid if we sustitute m N \ {1} y m N p k y u k and inary y decimal If m = 1 then (0) m 1 is defined as the empty word Lemma 12 holds when we sustitute p k y u k For any m N the list numer 0 M decimal generates a tree When we choose m = 2 the tree looks like this: (066) (066) (114477) (225588) (066) (114477) (225588) (770) (144881) (255992) (74074) (185185) (296296) (1467) (124579) (25690) The length of the period u k is regular for k 2 (i e l k+1 = l k ) For p prime the length of the period of 1/p k is usually (p 1)p k 1 There are three known exceptions and p = is one of them 4 We have 1/9 = 0111 and u 2 = 1 At level k = 2 only the word 11 really appears in 1/9 Therefore it is convenient to use the sutree generated y 11 ecause 11 generates the first triad ( 7 70) We have 1/27 = so u = 070 for m = 2 In u there are the three words 0 7 and 70 corresponding to the triad ( 7 70) at level k = Further 1/81 = Then u 4 = for m = 2 There are nine words in u 4 corresponding to the triads (1 4 67) ( ) and ( ) at level k = 4 4 Wikipedia electronic: decimal

16 INTEGERS: 12 (2012) 16 Proof of Theorem 7 For any m N define 1 m := m 1 j=0 10j Instead of Proposition 14 and Proposition 15 we have now { 1m 1m + 10 m 1m m } X m () = and (6) X m (k + 1) = n X m(k) { n n + 10 m n m } (k ) (7) It follows that the list numers of X m (k) (k ) have the form { 1m + t 10 m } X m (k) = t {0 1 2 k 2 k 2 1} We apply Proposition 2 paragraphs 1 and 2: X m (k) = M decimal if and only if 0 < 10 m / k 2 < 1 This condition yields k 0 = m ln(10)/ln() +2 Additionally m (k 2)ln()/ln(10) The numer of leading zeros in 1/ k is z = k ln()/ln(10) so that m 1 < z Finally f(n) f(n ) 1 follows directly from Proposition 2 paragraph The proof of Corollary 8 is almost identical to that of Theorem 2 Instead of Since at least one of three consecutive list numers say z must have its last inary digit equal to 1 in the last part of the proof of Theorem 2 we have now Since at least two of three consecutive list numers say z must have its last digit different from 0 Oviously instead of M inary Proposition 14 and Proposition 15 we have now M decimal expression (6) and expression (7) 6 A Helpful Sequence Lemma 21 Let a/ Q e a positive fraction in lowest terms: a N N \ {1} and gcd(a ) = 1 Let ( t a ) 1 t=0 e a finite sequence of non-negative integers where t B := {0 1 1} Then the following hold: If 0 < a < 1 then (t + 1) a t a 1 for all t t + 1 B; If 1 < a then there is some t B such that (t + 1) a t a 2 Proof Note that 2 1 Let 0 < a/ < 1 By the inequality x + y x + y + 1 for x y R we have t a + a t a + 1 for all t t + 1 B 2 Let 2 < a/ Then t 0 = 0 is the claimed t ecause a/ 0 2 Let 1 < a/ < 2 Then 1 a < 1 since a 1 We write a = 1 + a So t a = t + t a (8)

17 INTEGERS: 12 (2012) 17 There is a unique t 0 B such that t 0 (a ) + 1 = s is an integer ecause ta (mod ) has the solution t 0 1 a (mod ) We prove that t 0 B is the claimed t We have t0 (a ) + 1 = t 0(a ) + 1 = s; hence t0 (a ) = s 1 Furthermore t0 (a ) We apply relation (8): Finally + a t0 (a ) = (t 0 + 1) a = t t 0 a a 1 = s + a t 0 + a a = t 0 + t 0 (t 0 + 1) a a t 0 = 2 a 1 = s = t s = t 0 + s 1 Lemma 22 Let a/ Q e a positive fraction in lowest terms: a N N \ {1} and gcd(a ) = 1 Let A := {0 1 2 a 1} B := { } and n A Then { ta + n ta + n } { 0 D floor := t B = } Proof As a is relatively prime to the positive integer a generates the additive cyclic group of order in the finite ring Z/Z So {(ta) mod t B} = B For a fix n A ((ta + n) mod ) 1 t=0 permutes the elements of B Then also {(ta + n) mod t B} = B Finally { (ta + n) mod D floor = } t B = { } Proposition 2 Let a/ Q e a positive fraction in lowest terms: a N N \ {1} and gcd(a ) = 1 Let A := {0 1 2 a 1} B := { } and n A Then the following hold: 1 { t a } t B = A 0 < a < 1

18 INTEGERS: 12 (2012) 18 2 { ta + n } t B = A { t a } t B = A Let 0 < a < 1 and r := mod a In the sequence ( ta + n ) 1 S n := t=0 each element of A occurs either /a or /a times There are exactly r elements of A occurring /a times in S n ; each of the remaining a r elements occurs /a times Proof 1 By Lemma 21 2 We define ( ta + n ) 1 S n := t=0 By Lemma 22 the sequence S n contains exactly one integer We calculate the corresponding t solving ta + n 0 (mod ) So t 1 an (mod ) This linear function yields a unique t B for each n A and different t s for different n s We denote this special t y t n By Lemma 22 the sequence S n contains exactly one term (ta + n)/ such that (ta + n)/ + 1/ is integer We calculate the corresponding t solving ta + n (mod ) So t 1 a n 1 a (mod ) We denote this special t B y t n Note that t n t n 1 a (mod ) We get S n+1 (n a 1) y adding 1/ to all terms of S n so that an integer is generated in S n+1 at the position t n of S n and nowhere else y Lemma 22 Hence t n+1 = t n (n a 1) It follows that the sequences S n and S n+1 have the same terms excepted at the position t n+1 = t n: tn+1 a + n + 1 = t n+1a + n + 1 = t na + n + 1 t = n a + n + 1 Let { ta/ t B} = A Note that t 0 = 0 and t 0 t 0 Further t n 0 for all n A \ {0} ecause 1 a n 1 a 0 (mod ) yields n = 1 ut 1 A The sequence S 0 has two or more leading zeros When going up from n to n + 1 no integer gets lost Indeed let u n := (t n 1)a + n and v n := t na + n e two consecutive terms in S n Then v n + 1/ := s + 1 is an integer the only one in S n+1 The integer v n = s does not get lost in S n+1 since u n = s: (t n 1)a + n t = n a + n a = s a = s Conversely if { (ta+n)/ t B} = A for some n A then there is at least one integer s S n occurring twice (or more times): u n = v n = s at the positions

19 INTEGERS: 12 (2012) 19 t n 1 and t n Since t n = t n 1 we can go down from n to n 1 without loosing any of the integers of S n If n = a 1 then t a /a (mod ) It follows t a 1 1 (mod ) so the integers at the positions 2 and 1 in S a 1 are always equal There is a q N such that = qa + r and 0 < r < a Clearly q = /a For an interval I = [i i + 1) i A we have 1 = q a + r and 0 < r < a Let D floor e as in Lemma 22 Then {0/ 1/ 2/ (r 1)/} D floor Note that the interval I is closed at the left and open at the right For every r {0/ 1/ 2/ (r 1)/} the corresponding interval I corresponding to a t such that r = (ta + n)/ (ta + n)/ contains /a times the closed interval of length a/ and for any other r D floor only ( /a 1) times Note that q intervals have q + 1 endpoints Consequently there are r different integers in A each of them occurring exactly ( /a + 1) times in S n ; each of the remaining a r integers occurs exactly /a times References [1] D J Bernstein A non-iterative 2-adic statement of the x+1 conjecture Proc Amer Math Soc 121 (1994) Availale at [2] D J Bernstein and J C Lagarias The x + 1 conjugacy map Canadian Journal of Mathematics 48 (1996) Availale at lagarias [] J C Lagarias The x + 1 prolem and its generalizations Amer Math Monthly 92 (1985) 2 Availale at lagarias [4] J C Lagarias x + 1 prolem and related prolems Availale at lagarias [5] Josefina Lopez and Peter Stoll The x+1 conjugacy map over a Sturmian word Integers 9 (2009) A Availale at [6] M Lothaire Algeraic Cominatorics on Words volume 90 of Encyclopedia of Mathematics and its Applications Camridge University Press Camridge (2002)

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