A horizontal row comprising of squares has red counters placed at one end and blue counters at the other end, being separated by a single empty square in the centre. For example, when .
A counter can move from one square to the next (slide) or can jump over another counter (hop) as long as the square next to that counter is unoccupied.
Let represent the minimum number of moves/actions to completely reverse the positions of the coloured counters; that is, move all the red counters to the right and all the blue counters to the left.
It can be verified , which also happens to be a triangle number.
If we create a sequence based on the values of for which is a triangle number then the first five terms would be:
, , , , and , and their sum would be .
Find the sum of the first forty terms of this sequence.
Write-up coming later
The complete problem is available here. An approach, code, and answer will be added later.