Maths Olympiad Prep

Track / Stage 6 / 111 of 400 #1111 of 1964

Problem 1111

National olympiad, first round
Number theory Difficulty 6.1 Prove it

1.22*. (Jury, SFRY, 79). Let h(n)h(n) denote the largest prime divisor of the number nN(n2)n \in \mathbb{N} (n \geqslant 2). Is the set of values of nn satisfying the condition

h(n)<h(n+1)<h(n+2) h(n)<h(n+1)<h(n+2)

infinite?

This one wants a proof. Work it on paper, then read the official solution and mark yourself. Be honest about it: the record is only any use to you if it is.

Official solution

1.22. Let an odd prime number pp be chosen. We will prove that any two numbers from the increasing sequence of even numbers am=p2m+1(mZ+)a_{m}=p 2^{m}+1\left(m \in \mathbf{Z}^{+}\right) do not have common divisors greater than two. Indeed, if m>l0m>l \geqslant 0, then the number

pp2m1=(p2m1+1)(pm11)==(p2m1+1)(p2m2+1)(pl+1)(p2l1) \begin{aligned} p^{p 2^{m}}-1=\left(p 2^{m-1}+1\right) & \left(p^{m-1}-1\right)=\ldots \\ & \cdots=\left(p^{2 m-1}+1\right)\left(p 2^{m-2}+1\right) \ldots\left(p^{l}+1\right)\left(p 2^{l}-1\right) \end{aligned}

is divisible by p2+1p^{2}+1, therefore

(am,al)=(2+(p2m1),p2+1)=(2,p2+1)=2. \left(a_{m}, a_{l}\right)=\left(2+\left(p 2^{m}-1\right), p^{2}+1\right)=\left(2, p^{2}+1\right)=2 .

Consider the set of those members of the sequence that are divisible by at least one prime number greater than pp. This set is not empty, because, as a result of the property of the sequence {am}\left\{a_{m}\right\} proven above, only a finite number of its members can lack divisors greater than pp. Let ama_{m} be the smallest number in this set. Then we have

h(am)>p,h(am1)=h(p2m)=p h\left(a_{m}\right)>p, h\left(a_{m}-1\right)=h\left(p^{2 m}\right)=p

h(am2)=h(p2m1)=max{h(pm1+1),h(pm11)}= h\left(a_{m}-2\right)=h\left(p^{2^{m}}-1\right)=\max \left\{h\left(p^{m-1}+1\right), h\left(p^{m-1}-1\right)\right\}=\ldots

=max{h(pm1+1),h(p2mz+1),,h(p+1),h(p1)}= \ldots=\max \left\{h\left(p^{m-1}+1\right), h\left(p 2^{m-z}+1\right), \ldots, h(p+1), h(p-1)\right\}=

=max{h(am1),h(am2),,h(a0),h(p1)}<p, =\max \left\{h\left(a_{m-1}\right), h\left(a_{m-2}\right), \ldots, h\left(a_{0}\right), h(p-1)\right\}<p,

since h(p1)<ph(p-1)<p and for each value of t=0,1,,m2t=0,1, \ldots, m-2, m1m-1, according to the choice of the number mm, we have h(al)ph\left(a_{l}\right) \leqslant p, and moreover, al1(modp)a_{l} \equiv 1(\bmod p), from which h(al)<ph\left(a_{l}\right)<p. Thus, for a given odd prime number pp, a number of the form n=p2m1n=p^{2^{m}}-1 is specified, satisfying the inequalities

h(n)<h(n+1)<h(n+2). h(n)<h(n+1)<h(n+2) .

Since different numbers pp will yield different values of nn_{\text {; }}, the set of such values is infinite.

Source: NuminaMath-1.5, licensed Apache-2.0. Statement and solution reproduced as published; topic, difficulty and ordering added by this site.