What's the simplest rational not expressible as a sum of a given number of unit fractions? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T18:53:49Z http://mathoverflow.net/feeds/question/33628 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/33628/whats-the-simplest-rational-not-expressible-as-a-sum-of-a-given-number-of-unit-f What's the simplest rational not expressible as a sum of a given number of unit fractions? Gerry Myerson 2010-07-28T07:22:29Z 2010-09-14T11:43:36Z <p>This is essentially the same as the closed question <a href="http://mathoverflow.net/questions/32956/representation-of-rational-numbers-as-the-sum-of-1-k" rel="nofollow">http://mathoverflow.net/questions/32956/representation-of-rational-numbers-as-the-sum-of-1-k</a> but I hope I can make a case for it as an MO-worthy question. </p> <p>Ed Pegg, Jr., in his Math Games column for 19 July 2004 at the MAA website, <a href="http://www.maa.org/editorial/mathgames/mathgames_07_19_04.html" rel="nofollow">http://www.maa.org/editorial/mathgames/mathgames_07_19_04.html</a> 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?" </p> <p>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? </p> <p>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 $s$)? </p> <p>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. </p> <p>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 $${1\over2},{2\over3},{4\over5},{8\over11},{16\over17},{77\over79},{732\over733}.$$ </p> <p>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 $C>0$. </p> <p>At this point I will gladly settle for a calculation of $s(8)$. </p> http://mathoverflow.net/questions/33628/whats-the-simplest-rational-not-expressible-as-a-sum-of-a-given-number-of-unit-f/38675#38675 Answer by Hugo van der Sanden for What's the simplest rational not expressible as a sum of a given number of unit fractions? Hugo van der Sanden 2010-09-14T11:43:36Z 2010-09-14T11:43:36Z <p>$s(8) = \frac{27538}{27539}$.</p> <p>I have made the code I used available at <a href="http://crypt.org/hv/maths/least_eg-0.01.tar.gz" rel="nofollow">http://crypt.org/hv/maths/least_eg-0.01.tar.gz</a>, with a README file at <a href="http://crypt.org/hv/maths/least_eg-0.01-README" rel="nofollow">http://crypt.org/hv/maths/least_eg-0.01-README</a>.</p> <p>The package includes both PARI/GP code and C code using the GNU GMP library to calculate the results, as well as a synopsis of the results for each denominator from 2 to 27539 which may be of use for further analysis.</p> <p>I estimate the PARI code would have taken about a CPU-year to find the result; the C code runs over 20 times faster on my machine, and I don't understand why the difference is so great. (I'd appreciate email if someone can explain.)</p>