MathDB
Infinite sequence with weird conditions

Source: Junior Olympiad of Malaysia Shortlist 2015 N1

July 17, 2015
number theory

Problem Statement

Prove that there exists an infinite sequence of positive integers a1,a2,... a_1, a_2, ... such that for all positive integers i i , \\ i) ai+1 a_{i + 1} is divisible by ai a_{i} .\\ ii) ai a_i is not divisible by 3 3 .\\ iii) ai a_i is divisible by 2i+2 2^{i + 2} but not 2i+3 2^{i + 3} .\\ iv) 6ai+1 6a_i + 1 is a prime power.\\ v) ai a_i can be written as the sum of the two perfect squares.