MathDB
HMMT Feb 2023 team p6

Source:

February 20, 2023

Problem Statement

For any odd positive integer nn, let r(n)r(n) be the odd positive integer such that the binary representation of r(n)r(n) is the binary representation of nn written backwards. For example, r(2023)=r(111111001112)=111001111112=1855r(2023)=r(111111001112)=111001111112=1855. Determine, with proof, whether there exists a strictly increasing eight-term arithmetic progression a1,,a8a_1, \ldots, a_8 of odd positive integers such that r(a1),,r(a8)r(a_1), \ldots , r(a_8) is an arithmetic progression in that order.