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n-staircase of unit squares

Source: Canadian Mathematical Olympiad - 2010 - Problem 1.

May 6, 2011
inductiongeometrygeometric transformationreflectionsymmetrycombinatorics unsolvedcombinatorics

Problem Statement

For all natural nn, an nn-staircase is a figure consisting of unit squares, with one square in the first row, two squares in the second row, and so on, up to nn squares in the nthn^{th} row, such that all the left-most squares in each row are aligned vertically. Let f(n)f(n) denote the minimum number of square tiles requires to tile the nn-staircase, where the side lengths of the square tiles can be any natural number. e.g. f(2)=3f(2)=3 and f(4)=7f(4)=7. (a) Find all nn such that f(n)=nf(n)=n. (b) Find all nn such that f(n)=n+1f(n) = n+1.