Problem:
In a sequence of numbers, a term is called golden if it is divisible by the term immediately before it. What is the maximum possible number of golden terms in a permutation of ?
Problem:
In a sequence of numbers, a term is called golden if it is divisible by the term immediately before it. What is the maximum possible number of golden terms in a permutation of ?
Solution:
Let be the number of golden terms. We claim that .
Proof: Define the term immediately before a golden term to be a silver term. The number of silver terms is also . If is any silver term and is the corresponding golden term then we must have
Therefore . Hence and therefore there can be at most 1010 different silver terms. Therefore .
Now we need to show that is indeed possible. To do this we first partition the set into the following distinct parts:
Each part starts with an odd number, which is then doubled until the result is larger than 2021.
Putting each part in the above sequence in order is our example. In this example every even term, , is golden because it occurs immediately after the term . There are 1010 even numbers, so is possible.