# What's the simplest rational not expressible as a sum of a given number of unit fractions?

This is essentially the same as the closed question Representation of rational numbers as the sum of 1/k but I hope I can make a case for it as an MO-worthy question.

Ed Pegg, Jr., in his Math Games column for 19 July 2004 at the MAA website, http://www.maa.org/editorial/mathgames/mathgames_07_19_04.html writes, "Here is an interesting sequence of fractions that would likely would [sic] have fascinated Ahmes: $$1/2, 2/3, 4/5, 8/11, 14/17, 19/23, 24/29, 49/59, 65/71, 76/83, 61/157, 183/191, 260/269, 289/299.$$ $8/11 = 1/2 + 1/6 + 1/21 + 1/77$. This is the simplest Egyptian fraction that requires 4 parts. $14/17 = 1/2 + 1/4 + 1/20 + 1/55 + 1/187$ requires 5 parts. 289/299 is the simplest fraction that requires 14 parts. One might think that this sort of thing was well known, but it isn't.... What is the simplest fraction that requires 15 parts, 16 parts, and beyond?"

Pegg never defines "simplest," but presumably it means smallest (positive) denominator and, among fractions with the same denominator, smallest (positive) numerator. So the general question would be, given $s$, what's the simplest rational that can be expressed as a sum of $s$ unit fractions, but not fewer?

In this form, it's probably an open, and maybe impossible, problem (that is, I don't think anyone will find a simple formula for the rational as a function of $s$), so let me ask a bit less. Has there been any advance beyond 14 since 2004? Are there any bounds in the literature (that is, bounds on the "complexity" of the rational as a function of $s$)?

I note that Pegg gives no source for his list of 14. The Online Encyclopedia of Integer Sequences does not recognize the sequence of numerators, nor the sequence of denominators. Before anyone suggests typing "Egyptian fractions" into Google, or looking at the Wikipedia article on that subject, I hope he or she will verify that the particular question I'm asking is in fact answerable by such means.

EDIT: As per the comments, it appears that only the first four terms in Pegg's list are correct, and that the current state of knowledge is $${1\over2},{2\over3},{4\over5},{8\over11},{16\over17},{77\over79},{732\over733}.$$

Also as per the comments, if we are after $f(s)=\min\lbrace b:N(a,b)=s{\rm\ for\ some\ }a,1\le a\lt b\rbrace$ then $f(s)\ge e^{Cn^2}$ for some $C>0$, and, conjecturally, $f(s)\ge e^{e^{Cn}}$ for some $C>0$.

At this point I will gladly settle for a calculation of $s(8)$.

• 14/17 = 1/2 + 1/4 + 1/14 + 1/476 appears to be an error; I believe 16/17 = 1/2 + 1/3 + 1/10 + 1/128 + 1/32640 is the simplest requiring 5 terms. – Hugo van der Sanden Jul 28 '10 at 10:52
• "He conjectures that the true result is $N(a,b)=O(\log\text{}\log b)$, and he in fact establishes the inequalities $$\sum_{a=1}^{b-2}N(a,b)>{\textstyle\frac 1{2}}(b-2)(\log\text{}\log b-1),\quad N(b-1,b)>\log\text{}\log b-1.$$ " – Wadim Zudilin Jul 28 '10 at 12:23
• I haven't checked the intervening numbers, but by hand I found 289/299 = 1/2 + 1/3 + 1/8 + 1/156 + 1/552. I checked also just using the greedy algorithm, and that gives 1/2 + 1/3 + 1/8 + 1/122 + 1/39795 + 1/1935522680. So I have no idea what Pegg's numbers are supposed to represent, but I can't see any relation between them and the stated problem. Time permitting I'll try calculating the sequence, but I anticipate the correct denominators will grow much more rapidly, so it'll be hard to calculate more than 7-8 terms. – Hugo van der Sanden Jul 28 '10 at 18:44
• My apologies to all for taking Pegg's numbers on faith. Guy, Unsolved Problems In Number Theory, D11, writes, "Victor Meally...noted that 2/3, 4/5 and 8/11 are the [simplest rationals] that need 2, 3 and 4 [unit fractions].... Stephane Vandemergel, in a 93-04-28 letter, states that 16/17 requires 5 [unit] fractions, and 77/79 needs 6." With these numbers, I found oeis.org/A097048 which gives 732/733 as the next, and last known, term. So unless there has been some advance since then, exact values are only known to $s=7$. As for bounds... (see next comment) – Gerry Myerson Jul 29 '10 at 0:47
• Lasse: re: "Is it obvious / known that for any n there is a rational number that can be written as a sum of n unit fraction, but not of n-1?" Maybe not obvious, but it is known. If s_i is the i'th number in Sylvester's sequence then (s_i-1)/s_i requires i terms. E.g. 1/2 requires 1 term, 2/3 requires 2, 6/7 requires 3, 42/43 requires 4, etc. – David Eppstein Jul 29 '10 at 3:57

$s(8) = \frac{27538}{27539}$.