Puzzle Archive

December 2019 Solution

The text of the poem in this month’s puzzle suggests that the puzzle has something to do with chess. The first step to solving it is noticing that each of the 8 lines of the poem contains 8 words. The poem itself represents a chessboard, where each word that begins with K, Q, R, B, N, or P represents a king, queen, rook, bishop, knight, or pawn.

Five of the lines are capitalized and three are not; as in FEN notation, the capitalized lines contain white pieces and the un-capitalized lines contain black pieces. The poem asks for the fastest way to win, but notes that the history will be needed to prove that it’s actually the fastest way to win.¬† White could win in 2 moves, but only if black cannot castle. Can we determine from the current state of the board whether black is still able to castle? We will show that black cannot castle by analyzing the chess game itself.

There is no possibility for en passant so we know everything relevant about the board except whether black can castle.
We know white’s queen must be a promoted h pawn, since the original queen must have been taken by a knight without moving.
Given this, and black’s pawn structure, we know that white must have promoted this pawn on b8, c8, d8, e8, or f8.
If white promoted on d8, e8, or f8, then the black king must either move or already have moved.
If white had promoted on b8, it would have needed to capture all 6 of black’s missing pieces in order to travel that far to the left; however, it cannot possibly have captured black’s bishop in cell c8 which always stays on a different color.
If white had promoted on c8, it must have done so from square c7, having captured black’s bishop on square c8 previously via other means. It also must have done so after all of black’s pawn moves, since black’s bishop on f8 has to have come out, and since black’s a pawn must already have promoted.
Only a rook could be in square d8 to prevent the black king from having to move after the promotion on c8, but if that were the case, then on the move after the promotion black could only have moved the king or the king’s side rook.

Thus, we can conclude that black has already lost its option to castle, and white can mate in 2 with Rxd7 followed by Qb8#

Several solvers interpreted the ” — ” in line 5 of the poem to indicate that castling wasn’t available, as a dash in FEN notation will indicate that castling is unavailable. This was unintentional on our part!

Grading this month’s puzzle was a challenge but we generally gave credit to submitters who acknowledged that black could not castle, and found this 2 move checkmate as a result.

Congratulations to everyone who solved this month’s puzzle!

Correct Submissions from:
Milos Mitrovic and Uros Dinic
Sean Egan
Problem Solving Bois
Rafah Hajjar & Jordi Rodríguez
Ben Reiniger
Magic Mister
Scott Okuno
John Coltrane
Tomer Tzadok
Dimas Ramos, Glauber Guarinello & Rodrigo Amorim
Christopher Kei
Guillaume Aubian
Zeus Kronion
Reyer Swengel & Nadia Magnuson
Senthil Rajasekaran
Gil & Josh
Sebastian Lopez
Vinay Kameswaran, Shawn Ng, Dave Cox