Maths Olympiad Prep

Track / Stage 6 / 34 of 400 #1034 of 1964

Problem 1034

National olympiad, first round
Number theory Difficulty 6.0 Prove it

10.7. Around a circle, there are 10100010^{1000} natural numbers. Between every two adjacent numbers, their least common multiple is written. Can these least common multiples form 10100010^{1000} consecutive numbers (arranged in some order)?

(S. Berlov)

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

Answer: No.

Solution. Let n=101000n=10^{1000}. Denote the original numbers (in order of traversal) as a1,,ana_{1}, \ldots, a_{n}; we will assume that an+1=a1a_{n+1} = a_{1}. Let bi=LCM(ai,ai+1)b_{i} = \operatorname{LCM}(a_{i}, a_{i+1}). Suppose that the numbers b1,,bnb_{1}, \ldots, b_{n} are nn consecutive natural numbers.

Consider the highest power of two 2m2^{m} that divides at least one of the numbers aia_{i}. Note that none of the numbers b1,,bnb_{1}, \ldots, b_{n} is divisible by 2m+12^{m+1}. Let, for definiteness, a12ma_{1} \vdots 2^{m}; then b12mb_{1} \vdots 2^{m} and bn2mb_{n} \vdots 2^{m}. Thus, b1=2mxb_{1} = 2^{m} x and bn=2myb_{n} = 2^{m} y for some odd xx and yy. Without loss of generality, we can assume that x<yx < y. Then, since b1,,bnb_{1}, \ldots, b_{n} form nn consecutive numbers, among them there must be the number 2m(x+1)2^{m}(x+1) (since 2mx<2m(x+1)<2my2^{m} x < 2^{m}(x+1) < 2^{m} y). But this number is divisible by 2m+12^{m+1} (since x+1x+1 is even), which is impossible. Contradiction.

Comment. Only the answer - 0 points.

Proved that among the original numbers there are two numbers divisible by 2k2^{k} (the number kk is defined in the solution) - 2 points.

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