## Monday, April 11, 2022

### The Roeder Seq Problems was Solved Before I Posed it (Math)

(Joint Post by Bill Gasarch, David Harris, and Tomas Harris)

The divisor graph D(n) is an undirected graph with

vertex set V={1,...,n}\$ and

edge set E={(a,b) :  a  divides  b  or  b  divides  a }

We denote the length of the longest simple path in D(n) by L(n).

EXAMPLE: if n=10 then one long-ish sequence is

1,8,4,2,6,3,9

so L(10) GE 7. I leave it to the reader to do better OR to show its optimal.

In 2017 Oliver Roeder asked for L(100) (see here) In a later post Roeder reported that Anders Kaseorg claimed L(100)=77 (see  here). Anders gave a sequence and claimed that, by a computer search, this was optimal. The column also claims that other people also claimed 77 and nobody got a sequence of length 78, so the answer probably is 77 (it is now known that it IS 77).  Roeder also mentions the case of n=1000 for which Kaseorg showed L(1000) GE 418. No nontrivial lower bounds are known.

In 2019 I (Gasarch) asked about asymptotic results for L(n)  (see my blog post here and my open problems column here.) I began working on it with David and Tomas Harris. David proved that

Omega( n/( (log n)^{1.68} )  LE  L(n)  LE  O( n/( (log n)^{0.79} ).

We also studied human-readable proofs that L(100) LE X for some reasonable X, though getting a human-readable proof for X=77 seemed impossible. We did get L(100) LE 83, in a human-readable proof. (Some commenters on my post to sketched a proof  that L(100) LE 83 and another that L(100) LE 80 as well.)

But it turned out that this problem had already been studied, predating Roeder's column. (This blog post is all about the math, bout the math, no treble.  My next post will be about how we didn't know the literature until our paper was close to being finished.)

In 1982 Pomerance showed L(n)  LE o(n) (see here). Pollington had earlier shown

L(n) GE ne^{polylog(n)};

however, the paper is not online and hence is lost to history forever. (If you can find an online copy please email me the pointer and I will edit this post.)

In 1995 Gerald Tenenbaum showed, in a paper written in French,  that there exists a,b such that

n/(log n)^a LE L(n) LE n/(log n)^b (see here).

More recently, in 2021, Saias showed, in a paper written in French, that

L(n) GE (0.3 - o(1)) n/log n (see here).

(ADDED LATER:  I got a very angry email telling me that the paper was in English and that I am a moron. It turns out that the abstract is in English but the paper is in French, hence the person who send the letter only read the abstract which explains their mistake.)

He conjectures that L(n)  SIM cn/log n where c is likely in the interval [3,7]. (Apparently, no other information is known about the relevant constant factors in the estimates.)

Interestingly, the work of Tenenbaum and Saias also demonstrates why the study of L(n)  is not an idle problem in recreational mathematics. The upper bounds come from results on certain density conditions for prime factorization of random integers. That is, given an integer x chosen uniformly at random from the range {1,..., n} with prime factorization p1 GE p2 GE ... one wants to show that, with high probability, the primes pi are close to each other in a certain sense. Most recent results on L(n) have been tied closely with improved asymptotic estimates for deep number theory problems.

Determining the value of L(100) (i.e., Roeder's problem) was mentioned in Saias's paper. He claims that L(100) = 77 was discovered by Arnaud Chadozeau, who himself has written a number of papers on other properties of D(n). Since this paper was in 2021 it was after Roeder's column; however, we believe that the different discoveries of L(100) are independent. The recent work around Roeder's column appears to be done independently from the extensive French-language literature on the topic.

The following problems are  likely still open:

a) Find L(n) exactly for as many n as you can.  This would clearly need a computer program.

A listing of L(n) for n = 1 ... 200, computed by Rob Pratt and Nathan McNew,

appears as OEIS #A337125. This also includes additional references.

b) Find human-readable proofs for upper bounds on L(n) (likely not exact) for as many

n as you can.

ADDED LATER: Gaétan Berthe emailed me

-----------------------------------------------------------------

I'm the author of the last comment on your article about Roeder Sequence , as your curious about the subject I can share what we've done with my friend Paul Revenant those last few years for fun.

It all started with a competition between our classmates (see here though note that its in French) for the 100 and 1000 cases, after a few months Paul using a MIP solver gurobi was able to found a solution of size 666, and last year by studying the structure of the 666 solution we were able, with the help of gurobi again, to prove that there was no 667 solution either.

Paul then achieve to find very probable value of the sequence for 1 to 1000 (we didn't automatize the proof of 666 but it should be doable). On my side I tried to look for good solutions for the 10000 case, again using gurobi and the structure that appeared in the solution of size 666. The structure enable to cut the problem in two subpart, so the search goes faster I was able to find a solution of size 5505.

So I would say that the two mains reasons we're able to prove optimality for high numbers as 1000 are:

- MIP solver such as gurobi are very powerful tools.

- The longest path in the divisor graph are highly structured.

I joined our informal proof of the 666 case (the solution at the end), what is interesting is to understand how the solution is composed of different blocks depending of the prime decomposition of the elements. I joined lower bound from 1 to 1000 computed by Paul, that are very likely to be optimal.

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He also emailed me

1)  a list of the numbers I call L(n) for n=1 to 1000. These have not been refereed though I think they are correct. The list is here

AND

2)  a PROOF that L(1000)\le 666 (and they HAVE a sequence of length 666, so L(1000)=666).

Again, not refereed, but you can read the proof yourself here WARNING- the proof is in ENGLISH, so you cannot use it to improve your mathematical French.

1. How do you know Pollington had shown that. Is it in the "There is a long path in the divisor graph" paper?

1. Pomerance cites an unpublished pre-print from Pollington.

2. I was emailed a reference to the Pollington paper, so its actually published:

Ars Combinatorics - Jan 1983

I don't have an online copy but I think one could get one:

https://www.researchgate.net/publication/266303640_There_is_a_long_path_in_the_divisor_graph

2. I had a competition with my classmates a few years back to solve the 1000 case, we ended up with a friend with a proof for the 1000 case to be 666, for 10000 the best score I have for now is 5505 (very unlikely to be optimal).

1. We need better tools to prove optimality.

Is the quest for such interesting or not?

Ramsey(5) haunts me: we dont know it, and very little (any?) math of interest has come out of the search.

3. None of the links are working for me - they all point to pages on draft.blogger.com.
However, I found the original Riddler column here:
https://fivethirtyeight.com/features/pick-a-number-any-number/

1. Fixed, thanks. It turns out that ALL of the links went to draftblogge so I had to fix all of them. A weird glitch in the system.