user26387: Single-check chess is a variant where the first person to give check wins. There is a website that claims it has been solved, with white winning in four moves; however, I cannot find the moves anywhere.

user2299: [FEN ""]

1. Nc3 e6 (1...d5 2. Nb5 Kd7 3. Nf3) 2. Ne4 Ke7 3. d3 f6 (3... Nf6 4. Bg5) 4. Bg5 fxg5 5. Qd2 Nf6 6. Qxg5

And it's check next move. Very far from the quickest or the most elegant solution, but this will do until somebody posts a good one.

user23397: If you trust Stockfish's evaluation on lichess.org, using the "three check" variant where both sides already have two checks, White wins in 5 moves with 1. e4, 1. e3, 1. Nc3, or 1. Na3, and loses otherwise.

26387: <p>Single-check chess is a variant where the first person to give check wins. <a href="https://www.chessvariants.com/winning.dir/singleck.html" rel="nofollow noreferrer">There is a website</a> that claims it has been solved, with white winning in four moves; however, I cannot find the moves anywhere.</p>
 2299: <pre><code>[FEN &quot;&quot;]

1. Nc3 e6 (1...d5 2. Nb5 Kd7 3. Nf3) 2. Ne4 Ke7 3. d3 f6 (3... Nf6 4. Bg5) 4. Bg5 fxg5 5. Qd2 Nf6 6. Qxg5
</code></pre>
<p>And it's check next move. Very far from the quickest or the most elegant solution, but this will do until somebody posts a good one.</p>
 23397: <p>If you trust Stockfish's evaluation on <a href="https://lichess.org/analysis/threeCheck/rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR_w_KQkq_-_0_1_%2B2%2B2#0" rel="nofollow noreferrer">lichess.org</a>, using the &quot;three check&quot; variant where both sides already have two checks, White wins in 5 moves with 1. e4, 1. e3, 1. Nc3, or 1. Na3, and loses otherwise.</p>
