Maths Olympiad Prep

Library / /3 of 7

Number theory Difficulty 6.0 AIME, harder Prove it Switzerland

Problem:
Let SS be a non-empty set of positive integers such that for any nSn \in S, all positive divisors of 2n+12^{n}+1 are also in SS. Prove that SS contains an integer of the form
(p1p2p2023)2023 \left(p_{1} p_{2} \ldots p_{2023}\right)^{2023}
where p1,p2,,p2023p_{1}, p_{2}, \ldots, p_{2023} are distinct prime numbers, all greater than 2023.

Solutions — 2

Solution 1

Solution:
Since SS is non-empty, it contains some integer aa. Since 12a+11 \mid 2^{a}+1, it follows that 1S1 \in S and thus 21+1=3S2^{1}+1=3 \in S. We call a pair (m,n)(m, n) of positive integers kk-valid if we have m,nSm, n \in S, nm2n+1n|m| 2^{n}+1 and m/nm / n is divisible by kk distinct primes. We will show inductively that for every k1k \geq 1 there is a kk-valid pair.

Lemma. If (m,n)(m, n) is kk-valid and pp is prime divisor of m/nm / n, then (pαm,pαn)\left(p^{\alpha} m, p^{\alpha} n\right) is kk-valid for all integers α0\alpha \geq 0.

Proof. Using LTE, we have vp(2pm+1)=vp(2n+1)+1v_{p}\left(2^{p m}+1\right)=v_{p}\left(2^{n}+1\right)+1 and hence pnpm2pn+1p n|p m| 2^{p n}+1. Also, since pnSp n \in S, we have pmSp m \in S. Because also pm/pn=m/np m / p n=m / n, it follows that (pm,pn)(p m, p n) is kk-valid. Repeating α\alpha times gives the desired result.

For k=1k=1, take (m,n)=(3,1)(m, n)=(3,1). Now assume (m,n)(m, n) is kk-valid for some k1k \geq 1 and let pp be a prime dividing m/nm / n. For any integer t1t \geq 1 we have
0<vp(2tn+1)=vp(2n+1)+vp(t)<n+logp(t) 0<v_{p}\left(2^{t n}+1\right)=v_{p}\left(2^{n}+1\right)+v_{p}(t)<n+\log _{p}(t)
Thus, the kk primes we know to divide m/nm / n make up at most a fraction
ppn+logp(t)<(m/n)ntk \prod_{p} p^{n+\log _{p}(t)}<(m / n)^{n} t^{k}
of 2tn+12^{t n}+1. Using the lemma, we can pick tt arbitrarily large such that (tm,tn)(t m, t n) is still kk-valid. It follows that 2tn+12^{t n}+1 is divisible by a prime qq that does not already divide m/nm / n, hence (tmq,tn)(t m q, t n) is k+1k+1-valid. We conclude the solution by picking a NN-valid pair (m,n)(m, n) where NN is large enough to ensure m/nm / n is divisible by at least 2023 primes greater than 2023. We then use the lemma to make sure the powers of all these primes are in the prime factorisation of mm are at least 2023 .

Solution 2

Solution:
We start by defining the following sequence: a1=1a_{1}=1 and an+1=2an+1a_{n+1}=2^{a_{n}}+1. For the same reason as in the first solution we have 1S1 \in S and therefore anSa_{n} \in S for all nn, so it's enough to show that an integer as in the problem statement will divide one of the ana_{n}. Now we also define rn=an+1/anr_{n}=a_{n+1} / a_{n}. If rn1r_{n-1} is an integer it's odd (because ana_{n} is odd) and we can factorise:
an+1=2an+1=(2an1)rn1+1rn1=(2an1+1)i=0rn11(1)n(2an1)i=ani=0rn11(1an)i a_{n+1}=2^{a_{n}}+1=\left(2^{a_{n-1}}\right)^{r_{n-1}}+1^{r_{n-1}}=\left(2^{a_{n-1}+1}\right) \sum_{i=0}^{r_{n-1}-1}(-1)^{n}\left(2^{a_{n-1}}\right)^{i}=a_{n} \sum_{i=0}^{r_{n-1}-1}\left(1-a_{n}\right)^{i}
and therefore:
rn=i=0rn11(1an)i r_{n}=\sum_{i=0}^{r_{n-1}-1}\left(1-a_{n}\right)^{i}
is an integer as well. Since r1=3r_{1}=3 is an integer we therefore get by induction that all rnr_{n} are integers and the equation above holds for all nn. Now assume we have a divisor dd of nn and look at the equation mod dd, we get rnrn1r_{n} \equiv r_{n-1}, therefore:
drndrn1 d\left|r_{n} \Longleftrightarrow d\right| r_{n-1}
If we apply this to some prime pp we realise that as soon as pp divides ana_{n} it divides not only rn1r_{n-1} but also rmr_{m} for all mnm \geq n, so vp(an)v_{p}\left(a_{n}\right) is increasing and will eventually be bigger than 2023. Now let panp \mid a_{n} and k=vp(rn)k=v_{p}\left(r_{n}\right); since vp(an+1)=vp(an)+k>kv_{p}\left(a_{n+1}\right)=v_{p}\left(a_{n}\right)+k>k and because pkrnp^{k} \mid r_{n} but pk+1rnp^{k+1} \nmid r_{n} we also get: pkrn+1p^{k} \mid r_{n+1} but pk+1rn+1p^{k+1} \nmid r_{n+1} so vp(rn+1)=k=vp(rn)v_{p}\left(r_{n+1}\right)=k=v_{p}\left(r_{n}\right). Apart of n+1n+1 this is also true (again by induction) for all m>nm>n. So if there was only a finite amount of primes among the divisors of ana_{n}, we would get that rnr_{n} is constant for all nn bigger than some n0n_{0}, which would imply exponential growth, but ana_{n} grows much faster than exponential, so we can conclude that at some point a new prime will appear, which completes the proof.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.