PALINDROMIC SUMS OF PROPER DIVISORS. Paul Pollack Department of Mathematics, University of Georgia, Athens, GA 30602, USA

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1 PALINDROMIC SUMS OF PROPER DIVISORS Paul Pollack Department of Mathematics, University of Georgia, Athens, GA 30602, USA Received:, Revised:, Accepted:, Published: Abstract Fi an integer g 2. A natural number n is called a palindrome in base g if its base g epansion reads the same forwards and backwards. Let sn = d n, d<n d be the sum-of-proper-divisors function. We show that for almost all that is, asymptotically 00% of natural numbers n, sn is not a palindrome in base g. We also show how to reach the same conclusion for several other commonly occurring arithmetic functions.. Introduction Fi an integer g 2. We say that a natural number n is a palindrome in base g or g-palindromic if its base g epansion reads the same forwards and backwards. In other words, if we write n = a 0 + a g + + a t g t, where each a i {0,,..., g } and a t > 0, then a i = a t i for i = 0,, 2,..., t. A simple counting argument shows that the number of integers in [, ] that are palindromic in base g has order of magnitude /2 for all. Given a naturally occurring integer sequence, one might ask about the frequency with which this sequence intersects the set of palindromes. This has been studied for the sequence of primes [, 5], linear recurrence sequences [, 4], and the sequences of kth powers for fied k = 2, 3,... [3]. Here we investigate this problem for the sequence {sn} n=, where sn := d n, d<n d is the sum-of-proper-divisors function. Our main theorem is that sn is g-palindromic only for a density zero set of natural numbers n. In fact, we prove a bit more. Definition. Let k be a positive integer. We say that the positive integer n is k-nearly-palindromic in base g if either n < g 2k, or n g 2k and the first k digits of n coincide with the reversal of the last k digits of n. It is clear that every palindrome is k-nearly-palindromic for each k =, 2, 3,.... Theorem. Fi g 2. Let k be an integer with k 2. The upper density of those n for which sn is k-nearly-palindromic is O g / log k. In the opposite direction from Theorem, palindromic values of sn are at least as frequent as the primes, for the trivial reason that sn = for all prime values of n. However, we do not know how to prove that in each base g, the function sn assumes infinitely many distinct palindromic values; for instance, we do not know how to do this when g = 0. This question is uninteresting if, e.g., g is prime, since then sg k is always palindromic. This would follow from the conjecture that all large even n can be written as a sum p + q, with p and q distinct primes a slight strengthening of Goldbach, since we can then arrange for spq = p + q + to coincide with any large odd number. But the eisting results on the Goldbach conjecture seem, even under the Generalized Riemann Hypothesis, to be too weak to say anything about our problem. One might wonder why we concentrate on the particular function sn. It turns out that for most of the other commonly occurring arithmetic functions, the situation is much simpler. In 3, we show that for each function f {σ, ϕ, λ, d, ω, Ω}, the set of n for which fn is palindromic is a set of density zero.

2 INTEGERS: Notation and conventions We continue to use σn = d n d for the usual sum-of-divisors function, ϕn = #Z/nZ for the Euler function, λn for the Carmichael λ-function giving the eponent of Z/nZ, dn = d for the number-of-divisors function, and Ω and ω for the functions counting the number of prime divisors, with and without multiplicities, respectively. We use O and o-notation, as well as the symbols,, and, with their usual meanings. Dependence of implied constants is indicated with subscripts. For > 0, we let log = ma{, log }, and we let log k denote the kth iterate of log. Note that with this convention, log k for every > Palindromic values of the sum-of-proper-divisors function For each real u, let Du = {n N : sn un}. I933, Davenport showed that the sets Du possess an asymptotic density for every real u [6]. Calling this density Du, he proved that Du is continuous everywhere and that lim u Du =. The following result is an analogue of Davenport s theorem for arithmetic progressions. Lemma 2. Let a and q be integers with q > 0. For each real u, let D a,q u = {n a mod q : sn un}. Then for all u, # D a,q [, ] D a,q u := lim /q eists, and D a,q is a continuous function of u with lim u D a,q u =. Proof. While the discussion so far has been phrased in terms of sn/n, everything is easily translated to be about the function σn/n, since σn/n = + sn/n. In particular, Davenport s theorem may be read as asserting the eistence of a continuous limiting distribution for σn/n, while Lemma 2 amounts to the claim that σn/n has a continuous distribution function when n is restricted to the progression a mod q. For a wide class of arithmetic functions, H. N. Shapiro showed that the eistence of a distribution function relative to N implies the eistence of a distribution function relative to each arithmetic progression a mod q [4, Theorem 5.2]. Shapiro s result applies in particular to σn/n. The continuity of the resulting distribution function follows immediately from the continuity of Davenport s function Du. A priori, one might epect q distinct distribution functions D a,q corresponding to the q different choices for a mod q. In fact, there is quite a bit of redundancy. Lemma 3. Let a, b, and q be integers with q > 0. If gcda, q = gcdb, q, then D a,q = D b,q. Essential to the proof of Lemma 3 is the following result from probability, which is one concrete embodiment of the method of moments. See, for eample, the tetbook of Billingsley [2, Theorems 30. and 30.2, pp ]. Lemma 4. Let F, F 2, F 3,... be a sequence of distribution functions. Suppose that each F i corresponds to a probability measure on the real line concentrated on [0, ]. For each k =, 2, 3,..., assume that µ k := lim u k df j u j eists. Then there is a unique distribution function F possessing the µ k as its moments, and F n converges weakly to F as n. Proof of Lemma 3. Rather than sn/n or σn/n, it is convenient in this proof to work instead with the function n/σn, which is universally bounded between 0 and. Thus, for each a and q, we define D a,q u = lim n #{n : n a mod q and /q σn u}.

3 INTEGERS: A quick calculation shows that for u 0, D a,q u = D a,q u +. So to prove the lemma, it suffices to show that D a,q = D b,q if gcda, q = gcdb, q. By Lemma 4, it is enough to prove that for every k N, the kth moments of D a,q and D b,q agree. Equivalently, it suffices to show that for each such k, lim n n a mod q k n σn n n b mod q n σn k = 0. Define an arithmetic function h so that n/σn k = d n hd for each n. Then n n a mod q k n σn n n b mod q k n = σn d hd n n a mod q d n n n b mod q d n. We are assuming that gcda, q = gcdb, q, so that a and b share the same set of common divisors with q. Hence, gcdd, q divides a if and only if gcdd, q divides b. It follows that the parenthesized difference of sums in is either a difference of empty sums, or is a difference of two sums both of which count numbers in [, ] belonging to a prescribed congruence class modulo lcm[q, d]. Hence, this difference of sums is always bounded by in absolute value. Thus, the proof of the lemma will be completed if can show that hd = o, d as. For primes p and positive integers e, we have hp e = p e /σp e k p e /σp e k, k which makes clear that hp e. Moreover, hp = p+ ; from the mean value theorem applied to t t k, we deduce that hp k p + < k p for each prime p. Since every d can be decomposed as the product of a squarefree number d and a coprime squarefull number d 2, hd hd hd 2 d d d squarefree d d squarefree /2 p k ωd d + k p d 2 d 2 squarefull d 2 d 2 squarefull k /2 log k. This is certainly o, and so the proof is complete. Lemma 5. Let W be a fied positive integer. Then W σn for all n outside of a set of asymptotic density zero. Proof. Watson, investigating unpublished claims of Ramanujan, showed that the number of n for which W σn is O/log /ϕw for 2 [5, Hauptsatz 2]. The implied constant in Watson s result may depend on W. Watson s theorem is enough for our present purposes, but we note that from [3, Theorem 2], one can deduce that the same O-estimate holds uniformly in W.

4 INTEGERS: If I is a bounded interval of the real line and F is a continuous distribution function, we let F I = F b F a, where a < b are the endpoints of I. The net lemma is a weak form of a theorem of Erdős [9, Theorem, p. 60]. Lemma 6. Let I be a bounded interval of the real line of length I > 0. Then for Davenport s distribution function D, we have DI / log2 + I. We can now prove our main result. Proof of Theorem. Let D k be the set of positive integers n for which sn is k-nearly-palindromic and let D k := D k [,. To prove the theorem, it suffices to show that lim sup m #D k g m g m g log k. We proceed to estimate the cardinality of D k g m for large m. D k g m, we may assume all of the following: When counting elements n i sn/n > /k, ii sn/n < k, iii n > g m / log k, iv σn 0 mod g k. v gcdn, g k g k 2ωg. Indeed, taking I = [0, /k] in Lemma 6 shows that the number of n g m violating i is Og m / log k for large m. Throughout the proof, the notion of large may depend on both g and k. Since n g sn/n m n g σn/n m gm d g m /d2 < 2g m, there are only Og m /k values of n g m violating ii. That we can assume iii is trivial, and that we can assume iv is immediate from Lemma 5. Now we turn to v. If v fails for n, then p e gcdn, g k > g k 2ωg, p e n p g and so n is divisible by some prime power p e > g k 2, where p g. Clearly, e >. In particular, n has a squarefull divisor eceeding g k 2 ; but the number of such n g m is O g g m / log k. Thus, v is safe to assume. For later use, we record that i and iii imply Now let Observe that since sn > n k > gm k log k gm l, where l := 2 log k. A = the integer formed by the first k digits of n, B = the integer formed by the last k digits of sn. n = σn sn sn B mod g k, the last k digits of n are determined by B. For large enough values of m, all n under consideration have sn > g m l > g 2k. Since sn is k-nearly-palindromic, the first k digits of sn are formed by reversing the digits of B. In particular, the last digit of B is not zero. Let B be the integer formed by reversing the digits of B. Then A g a n < A + g a, B g a+b sn < B + g a+b

5 INTEGERS: for certain integers a and b, and B A gb A + sn n B A gb + B. Since both B and A have k digits in base g, we see that B/A g. Now from i and ii, b = O g log k. Thus, [ sn B n A gb C kg k, B ] A gb + C 2 kg k 2 for certain constants C and C 2 depending only on g. Note that n has a+k digits, and so a m k, since n < g m. Since n > g m / log k g m l, we also have a > m k l. For fied A, B, a, and b, we estimate the number of n [A g a, A + g a having n B mod g k and satisfying 2. Let I denote the interval appearing on the right-hand side of 2. Then the number of these n is at most A + g a D B,g ki g k A g a D B,gkI g k + og r = D B,g kig a k + og r, 3 where the o-estimates are valid as m. Since n B mod g k, we have by v. From Lemma 3, d : = gcd B, g k D B,g ki #{M mod g k : gcdm, g k = d} = Thus, Now ϕ gk d = gk d = gcdn, g k g k 2ωg, 4 M mod g k gcdm,g k =d D M,g ki = g k lim #{n : gcdn, gk = d and sn/n I} g k DI. D B,g ki = g k #{M mod g k : gcdm, g k = d} DI g k ϕg k /d DI. p g k /d /p g gk d ; using this and our upper bound 4 on d, we obtain that D B,g ki g g k 2ωg DI g k 2ωg DI. Since I g kg k, Lemma 6 gives that DI g k. Hence, k 2ωg D B,g ki g. k Using this estimate in 3, and summing over a {m k l +, m k l + 2,..., m k}, we see that the number of n that arise from fied choices of A, B, and b is g g m 2k k2ωg up to an error term that is og m as m. Finally, we sum over the Og k possibilities for A, the Og k possibilities for B, and the O g log k possibilities for b. We conclude that k, as desired. lim sup m #D k g m k2ωg+ g m g + g log k k log k,

6 INTEGERS: Other arithmetic functions 3.. The functions ω, Ω, and d We need two results concerning the distribution of the number of prime factors of n for typical values of n. Lemma 7. Let K and let. The number of n for which ωn > K is O/K 2. The same estimate holds with ω replaced by Ω. Proof. This follows immediately from the theorem of Turán that for each of f = ω and f = Ω, we have n fn 2 = O. See, for eample, [7, pp ]. Lemma 8. Let. Then ma #{n : ωn = t} t log2, where the maimum is taken over all nonnegative integers t. replaced by Ω. The same theorem holds with ω Proof. When t = 0, there is precisely one integer n with ωn = t, namely n =. Suppose now that t. According to a theorem of Hardy and Ramanujan [0, Lemma A], #{n : ωn = t} log + c t, 5 t! for a certain absolute positive constant c. The right-hand side assumes its maimum value at t = + O, and a straightforward computation with Stirling s formula shows that its value there is O/. This handles the case of ω. Hardy and Ramanujan also proved the inequality 5 with Ω in place of ω under the restriction that t.9 see [0, Lemma C]. The above argument shows that #{n : Ωn = t} / for these t. Finally, for t >.9, the sharper bound #{n : Ωn = t} / follows from Lemma 7. It is easy to deduce from Lemmas 7 and 8 that ω and Ω are palindromic only on a set of n of density zero. It is only necessary to observe that the number of palindromes within log log 0.5 of log log is o and then to apply Lemma 8. We leave the details to the reader. Establishing a corresponding result for the number-of-divisors function dn requires somewhat more intricate arguments. Theorem 9. Fi an integer g 2, and assume that g is not a power of 2. For each k, the set of n for which dn is k-nearly-palindromic has upper density O g g 2k/3. Remark. If g is a power of 2, then the last k digits of dn are 0 in base g for almost all n. So the conclusion of Theorem 9 remains true, but for uninteresting reasons. We introduce one more piece of notation before embarking on the proof of Theorem 9. For each natural number n, we let ln denote the multiplicative order of 2 modulo n, where n is the largest odd divisor of n. Note that for integers h at least as large as the eponent of 2 dividing n, the residue class of 2 h modulo n depends only on the residue class of h modulo ln. Proof. The proof borrows some ideas from [4]. Write n = n 2, where is the largest squarefull divisor of n. Then n 2 is squarefree and gcdn 2, =. We may assume both of the following conditions: i g 4k/3, ii ωn 2 g k/3.

7 INTEGERS: Indeed, the number of n for which i fails is Og 2k/3. Now assume that i holds. Then Lemma 7 shows that the number of n 2 / for which ii fails is Og 2k/3 for large. The notion of large here is allowed to depend on both g and k. Summing on bounds the total number of n arising in this way by Og 2k/3. So the combined eceptions to i or ii make up a set contributing only Og 2k/3 to our upper density bound. We partition the remaining n into finitely many classes based on the value of the ordered pair, ωn 2 mod lg k. We will show that for each fied pair of this type, the number of corresponding n is, for large, g g 2k/3 lg k. 6 Summing over the lg k possibilities for ωn 2 mod lg k and then over squarefull g 4k/3 completes the proof of the theorem. Given a pair of this type, write the second component of the pair as R mod lg k, where 0 R < lg k. Once is large, i and ii show that ωn 2 is also large. Hence, 2 ωn2 is determined modulo g k by R, and the last k digits of dn = d 2 ωn2 are determined by and R. Let B denote these last k digits. Since dn 2 ωn2, we see that dn > g 2k once is large. We are assuming that dn is k-nearly-palindromic. So if B is the integer obtained by reversing the digits of B, then B is also a k-digit integer i.e., B does not end in zero and B gives the first k digits of dn. Choosing the integer s so that B g s dn < B + g s, we find that 0 < log dn s log B = log + / B We now look mod. Then these inequalities show that log dn g g k. belongs to an arc A n,r of the circle R/Z with length O g g k. Write ωn 2 = R + t lg k, where t is a nonnegative integer. From ii, we have t = t 0 + t, where 0 t 2g k/3 lg k, and t 0 := log dn g k/3 lg k R. belongs to A n,r modulo amounts to the Since dn = d 2 ωn2, the condition that requirement on t that log dn + R + t 0lg k + t lg k A n,r. Since g is not a power of 2, the number lg k is irrational, and a classical result of Weyl yields the uniform distribution of the sequence {t lg k } t =0 modulo. As a consequence, the discrepancy of the sequence {t lg k mod }m t =0 tends to 0 as m. It follows that the number of possibilities for t, and hence also for ωn 2 = R + t 0 + t lg k, is g g k/3 lg k g k = g 2k/3 lg k for large. Lemma 8 shows that the number of values of n 2 / corresponding to these possibilities for ωn 2 is g g 2k/3 lg k /n in eact agreement with 6. This completes the proof. = g 2k/3 lg k,,

8 INTEGERS: The functions ϕ, σ, and λ, and their compositions For these functions, we can establish strong results using nothing about palindromes other than the fact that they are relatively infrequent. Call a set S of natural numbers thin if for all large, the number of elements in S not eceeding is bounded above by / eplog c for some constant c = cs > 0. The following theorem was recently established by Vandehey and the author [2, Theorem 2]: Propositio0. Let f be any function of the form f f 2 f 3 f j, where j is a natural number, and each f i {ϕ, σ, λ}. Then f has the property that the inverse image of each thin set is also thin. This has the following consequence. Corollary. Let f be any of the functions considered in Propositio0. Then the set of n for which fn is a palindrome, or is a palindrome after all trailing zeros have been deleted, is a thin set. The corollary is immediate from Propositio0, since the set of integers that are palindromic in base g after trailing zeros have been removed has counting function O g /2. 4. A concluding remark Perhaps Theorem can also be established using nothing but the sparsity of the set of palindromes. Indeed, Erdős, Granville, Pomerance, and Spiro conjectured [8, Conjecture 4] that if A is any set of asymptotic density zero, then s A also has asymptotic density zero. Unfortunately, up to now nothing nontrivial in this direction has been shown without making further structural assumptions on A. References [] W. D. Banks, D. N. Hart, and M. Sakata, Almost all palindromes are composite, Math. Res. Lett. 2004, [2] P. Billingsley, Probability and measure, second ed., Wiley Series in Probability and Mathematical Statistics, John Wiley & Sons Inc., New York, 986. [3] J. Cilleruelo, F. Luca, and I. E. Shparlinski, Power values of palindromes, J. Comb. Number Theory 2009, [4] J. Cilleruelo, R. Tesoro, and F. Luca, Palindromes in linear recurrence sequences, Monatsh. Math , [5] S. Col, Palindromes dans les progressions arithmétiques, Acta Arith , 4. [6] H. Davenport, Über numeri abundantes, S.-Ber. Preuß. Akad. Wiss., math.-nat. Kl. 933, [7] J.-M. De Koninck and F. Luca, Analytic number theory: Eploring the anatomy of integers, Graduate Studies in Mathematics, vol. 34, American Mathematical Society, Providence, RI, 202. [8] P. Erdős, A. Granville, C. Pomerance, and C. Spiro, On the normal behavior of the iterates of some arithmetic functions, Analytic number theory Allerton Park, IL, 989, Progr. Math., vol. 85, Birkhäuser Boston, Boston, MA, 990, pp

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