# Retrograde Go Problem (Some Assembly Required)

An entry into the Fortnightly Topic Challenge #40.

In case you aren't familiar with the rules of go, the information in the basic rules section of the Wikipedia page should be enough to solve this puzzle. Anyway, here we go:

Given the following position, and this knowledge:

1. The game started on an empty 6x6 board
2. Exactly 5 moves have been played
3. There have been no passes

What was the last move black played?

Here's the position:

(Sorry, gotta run. Surely you can construct a suitable position for this problem yourself? Remember to make it solvable though!)

If it should so happen that there were more than one such position, the one where the stones fit inside the smallest rectangle is the winner.

• cannot see any photo – Oray Oct 16 '18 at 11:13
• @oray that’s the joke. – boboquack Oct 16 '18 at 11:14

I'm reasonably sure this is the best possible solution.

The most recent move must be

Black playing the corner, having captured White in the corner on their previous move.

I believe (without proof) that the only solution up to symmetry is:

(Yay for smartphone drawing skills!)

This is solvable since:

White must have played two stones, so one is captured: that must be on the square between the three blacks. But there couldn’t have been a black in the corner when the second white stone was played (it would have been taken) so the black stone in the corner must have been the last played.

• Very good! There is another, somewhat simpler solution though. – Bass Oct 16 '18 at 11:17
• I only just remembered that on this site multiple solutions are frowned upon, so I added a tie breaker to the question. Your solution has the tie breaker score 6 (2 times 3 intersections), which is pretty good. – Bass Oct 16 '18 at 12:54