MathDB
There exists a quadratic sequence with a_0 = 0, a_n = 1996

Source: IMO Shortlist 1996, N3

August 9, 2008
number theoryInteger sequencePerfect SquaresExtremal combinatoricsrecurrence relationIMO Shortlist

Problem Statement

A finite sequence of integers a0,a1,,an a_0, a_1, \ldots, a_n is called quadratic if for each i i in the set {1,2,n} \{1,2 \ldots, n\} we have the equality |a_i \minus{} a_{i\minus{}1}| \equal{} i^2. a.) Prove that any two integers b b and c, c, there exists a natural number n n and a quadratic sequence with a_0 \equal{} b and a_n \equal{} c. b.) Find the smallest natural number n n for which there exists a quadratic sequence with a_0 \equal{} 0 and a_n \equal{} 1996.