My guess is Richard Kaye, "Minesweeper is NP-complete", but it's paywalled. Certainly the paper is listed somewhere on https://minesweepergame.com/math-papers.

My guess is Richard Kaye, "Minesweeper is NP-complete", but it's paywalled. Certainly the paper is listed somewhere on https://minesweepergame.com/math-papers.php https://minesweepergame.com/math-papers.php