Magic Zero Hexagons
The numbers to 9 can be arranged in a hexagon so that all lines of hexes have a sum of zero. There seem to be 19 distinct ways to do this. By comparison, the numbers 1 to 19 can be arranged for a magic sum of 38 in a unique way; select arrangement 20 to see it.[more]
The triangular numbers are . This Demonstration shows that to make magic zero hexagons. Can to make a magic hexagon for all ?[less]
 Wikipedia. "Magic Hexagon." (Dec 4, 2020) en.wikipedia.org/wiki/Magic_hexagon.