Has an inventor found the hardest possible simple sliding-block puzzle?Sliding-block puzzles look easy, but they can be tricky to solve. The best known is the “15 Puzzle”, which became hugely popular in the late 1870s. This involves square tiles labelled with the numbers 1 to 15, which must be arranged in the correct order inside a four-by-four frame. Another popular one, called “Dad’s Puzzle”, involves moving a large square tile from one corner to another, by rearranging other, smaller tiles around it—akin to moving a piano across a cluttered room.
The best such puzzles are easy to explain, yet difficult to solve. Historically, they have been devised by trial and error. But earlier this year, Jim Lewis, an inventor based in Midland Park, New Jersey, set out to find the hardest possible “simple” puzzle, using a computer-based search.
More here in The Economist.