Apologies if my post was confusingly worded. I meant I detect two solutions to RWG's chess problem (at bottom): r3k3/p1p5/Q3K3/8/8/8/8/8 w . The Smullyan retrograde problem definitely has only one solution! —Dan

From: Bob Hearn <bob@hearn.to>

That is my very favorite retrograde problem. And I’m pretty sure there’s only one solution.

It’s the cover of Smullyan’s Chess Mysteries of the Arabian Knights: https://images-na.ssl-images-amazon.com/images/I/91B-xKpMIFL.jpg

”The White king, Haroun al Rashid, has made himself invisible. Where is he?”

(Incidentally Smullyan autographed my copy “I wish you the best, but I refuse to sign this book! Raymond Smullyan”.)

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On May 2, 2017, at 12:37 PM, Dan Asimov <dasimov@earthlink.net> wrote:

Cute!

(I detect two solutions; another piece could be added to prevent a second option.)

I'm still very fond of a retrograde chess problem from a Martin Gardner column, attributed to Raymond Smullyan:

. . . . . . . .

. . . . . . . .

. . . . . . . .

. . . . . . . .

. r . b . . . .

B . . . . . . .

. . . . . . . .

. . . . . . . .

. . . k . . . .

After a legal move has been completed in a legal chess game, the white king was accidentally knocked off the board.

Puzzle: Where was the white king?

(No fair if you've already seen this.)

—Dan

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From: Bill Gosper <billgosper@gmail.com>

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r3k3/p1p5/Q3K3/8/8/8/8/8 w --rwg