MathDB

C1

Part of 2020-IMOC

Problems(1)

N black triangles, One big triangle wanted

Source: 2020 IMOC C1

9/4/2020
Find all positive integer NN such that for any infinite triangular grid with exactly NN black unit equilateral triangles, there exists an equilateral triangle SS whose sides align with grid lines such that there is exactly one black unit equilateral triangle outside of S.S.
(ltf0501)
IMOCcombinatorics