Small prime factors of odd non-deficient numbers Joshua Zelinsky Hopkins School [email protected] Abstract Let σ(n) to be the sum of the positive divisors of n. A number is non-deficient if σ(n) 2n. We establish new lower bounds for the ≥ number of distinct prime factors of an odd non-deficient number in terms of its second smallest, third smallest and fourth smallest prime factors. We also obtain tighter bounds for odd perfect numbers. We also discuss the behavior of σ(n!+1), σ(2n+1), and related sequences. We will write σ(n) to be the sum of the positive divisors of n. Recall, that σ(n) is a multiplicative function. Recall further that numbers where σ(n)=2n are said to be perfect, numbers where σ(n) < 2n are said to be deficient, and numbers where σ(n) > 2n are said to be abundant. A large amount of prior work has been done on estimating the density of abundant numbers. See for example [7] and [8]. a1 a2 ak We will throughout this paper write n = p1 p2 pk where p1 pk are primes satisfying p < p < p . Bounds giving··· a lower bound··· for 1 2 ··· k n in terms of pi for either small i or i close to k exist already in the liter- ature, under the assumption that n is non-deficient or under the stronger assumption that n is an odd perfect number. Acquaah and Konyagin showed [1] that if N is an odd perfect number, arXiv:2005.12118v4 [math.NT] 7 Aug 2021 then 1 pk < (3N) 3 , (1) and 1 pk−1 < (2N) 4 . (2) Building on their work, the author [15] showed that 1 pk−1 < (2N) 5 , (3) and that 1 1 pk−1pk < 6 4 N 2 . (4) 1 However, equations 1 through 4 give a lower bound for N in terms of pk and pk−1. They do not give a lower bound for k, except in so far as we have upper bounds for N in terms of k, and the inequalities can then be chained together. In contrast, for pi, when i is small, the natural rout seems to go mainly in the other direction, using pi to bound k from below, and then use those results to obtain a lower bound on N. The main results of this paper are improved lower bounds on k in terms of p2 and p3 when n is odd and not deficient. We prove similar bounds with p4 when n is odd and perfect, and a slightly weaker result when n is a primitive non-deficient number. Using the same techniques we will also produce estimates for σ(m) when m is near but not equal to n!. Kishore [6] proved that if n is an odd perfect number with k distinct prime factors, and 2 i 6, then one must have ≤ ≤ i−1 p < 22 (k i + 1) . i − Note that these bounds are all linear in pi and k. In this paper, for i in the range 2 i 4, we prove improve on Kishore’s results, giving better than linear bounds.≤ ≤ Moreover, for i = 2 and i = 3, our results apply not just to odd perfect numbers but also to odd abundant numbers. For p1, better than linear bounds are known, due to [10] and the au- thor [14]. Most of the results in this paper rely on the same fundamental techniques as in [10] and [14]. The first section of this paper extends those techniques to these small prime divisors. The second section uses the same techniques to look at a related problem involving the sum of the divisors of numbers near n!. σ(n) We will write h(n)= n . Note that h(n) is frequently called the abun- dancy of n, but it has also been referred in the literature as the abundancy index of n as well as just the index of n. A number is perfect if and only if h(n) = 2, is abundant if and only h(n) > 2, and is deficient if and only h(n) < 2. One has that 1 1 h(n)= d = (5) n d Xd|n Xd|n from which it follows that h(mn) > h(n) (6) whenever m > 1. A consequence of 6 is that any multiple of an abundant number is abundant, and any multiple of a perfect number other than the number itself is abundant. Motivated by this consequence, there is a notion of a primitive abundant number which is a number n which is abundant but where proper all divisors of n are not abundant. There is some inconsistency in the literature here; some authors define primitive abundant numbers as 2 including perfect numbers where all proper divisors are deficient. In some respects, this is a more natural set to investigate. Others term this set to be the set of primitive non-deficient numbers, which is the term we will use. The first substantial work on this topic was Dickson who proved that given a fixed k, there are only a finite number of primitive non-deficient numbers with k distinct prime factors.[3] Prior work on these sets have looked at their density. See especially [2] as well as [9] and [13]. We will write H(n) = Πk pi . It is well-known and not hard to show i=1 pi−1 that h(n) H(n) (and this inequality is strict as long as n> 1). In fact, ≤ lim h(nm)= H(n). m→∞ Thus, H(n) is the best possible upper bound on h(n) when we know only the distinct prime divisors of n, but have no other information about n. We will write Pj as the jth prime number (note the capital letter dis- tinguishing this from the pi). Given a prime Pj and a real number α > 1, we will write bα(Pj) to be the smallest positive integer b such that P P P α< j j+1 j+b−1 P 1 P 1 ··· P 1 j − j+1 − j+b−1 − We will similarly define aα(j) as bα(Pj). These two functions, aα(j), and bα(Pj) are generalizations of the func- tions labeled as a(j), and b(Pj) in [10] and [14]. In particular, a(j)= a2(j) and b(j)= b2(j). The use of α = 2 arises naturally in the context when one is interested in bounding k in terms of the smallest prime divisor of n, but examining other prime divisors requires looking at other values of α. Note that if Pj is the smallest prime divisor of an odd perfect or or odd abundant number n then k a(j). Norton proved≥ Theorem 1. Let t be a positive integer then: t +1 5 1 1 a(t) > t2 2t . (7) − − log t − 4 − 2t − 4t log t 1 2 In fact, a(t) is asymptotic to 2 t log t. When Norton was writing, bounds on the prime number theorem and related functions were not strong enough to easily give an explicit lower bound on a(t) which had the correct lead term. Subsequently, the author proved a an explicit version with the correct asymptotic [14]: Theorem 2. For all t> 1 we have t2 3 1 log log t a(t) log t log log t + + t +1. (8) ≥ 2 − 2 20 log t − 3 Notice that this theorem has the correct first lead term. Norton also proved that 1 1 3 n2 log log n n2 a(n)= n2 log n + n2 log log n n2 + + O . (9) 2 2 − 4 2 log n log n Thus, Inequality 8 is not as strong as it could be although it has the correct lead term, the later terms do not match the asymptotic. In the final section of this paper we then use the same techniques to give a new proof of a theorem about the abundancy of numbers of the form m!+ z where z is a fixed non-zero integer. We will prove Theorem 3. Let z be a non-zero integer. Then lim h(m!+ z)= h( z ). m→∞ | | Theorem 3 appears to be a known folklore result; we are uncertain if it has been previously published. 1 Small prime factors of non-deficient num- bers We require the following explicit version of Mertens’ Theorem: Lemma 4. We have 1 p 1 eγ (log x) 1 < < eγ (log x) 1+ . (10) − 2 p 1 2 2 log x p≤x 2 log x Y − Here γ is Euler’s constant, with the lower bound valid for x > 3 and the upper bound valid for x 286. ≥ Proof. This is proven in [11]. Note that it is a consequence of this explicit version of Merten’s Theorem and the Prime Number Theorem that Theorem 5. Let α> 1 be fixed. Then pα b (p) , α ∼ α log p and nα log n a (n) . α ∼ α 4 Much of our work will be trying to make Theorem 5 explicit for certain specific values of α. We will for our purposes only need to construct explicit lower bounds, but it may be interesting to construct similarly explicit upper bounds. p We will write S(x)= p≤x p−1 , and will write Q p S(x) Q(x, y)= = p 1 S(y) y<p≤x Y − and will when we write this assume that x>y> 3. Lemma 6. Let A, B and α be real numbers.