This is an illegal checkmate
Illegal Position Contest!
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n9531l1 wrote:
daStrwbrry wrote:
I’ve used a similar pawn structure before, but the idea here is slightly different.
I don't have a proof, but wanted to see how close I could come. This almost works, but there's no way to have the bishops trade places.
It's deeper than that, even freeing up one the kings via a knight move trying to rearrange stuff so that the bishops had the b1 square, the black king would have to be on b2 which would make qa1+ impossible at the point the knight is still on b1 (since the king would have to move from a3 first to free up room for the knight.)
Can't remember if this was posted or not but I'm not sure about this position:
EndgameEnthusiast2357 wrote:
Can't remember if this was posted or not but I'm not sure about this position:
#8669
Simple enough cool!
daStrwbrry wrote:
#8668
Something like this almost works:
...but there's that 3rd Black Rook, and no way to have played say, ...f3xe2 to promote it. No more missing white units.
BigDoggProblem wrote:
daStrwbrry wrote:
#8668
Something like this almost works:
...but there's that 3rd Black Rook, and no way to have played say, ...f3xe2 to promote it. No more missing white units.
Indeed, you did find the “release” process which I thought was a bit challenging. Well done!
Another way of thinking about the illegality is that the last move must be Nc6xBa5, but because of the intricate release process and limited pieces available for captures, the recently uncaptured black DSB must uncapture a white rook at the dark squares e1 or g1, but it can’t get there.
Here are some more positions:
daStrwbrry wrote:
Here are some more positions:
#8675
Last move was Qe1xf1+
The white g-pawn made 4 captures. Black has 12 units after putting something back on f1; no spare uncaptures. Ph7 didn't capture. Thus, black must have made two captures [the two missing R's] to let it through.
f1 is a white square, so black's DSB must have been captured by the white g-pawn, on d6/e5/f4. [Not on c7, because then wP couldn't have captured the c-pawn.] This means the wP didn't pass through c6.
There is always one black major piece that cannot get back home. For example:
Or, in this case, the Q:
You can get both K and Q home, but only if you leave out Ra8 or Bf8 getting home.
Wonder if a computer program could be developed that could determine whether an inputted position is legal or not, using all the reasoning in this thread.
EndgameEnthusiast2357 wrote:
Wonder if a computer program could be developed that could determine whether an inputted position is legal or not, using all the reasoning in this thread.
It's possible, but difficult. Many of the problems have long term obstacles and conflicting goals, so it would be tough for the machine to know if it is making progress, or taking an extended trip to a brick wall.
daStrwbrry wrote:
#8676
Black is apparently missing 2P and 1 N. White made 3 captures.
White is missing LSB, which was captured on f1, and N, which must have been captured on the c-file. Black made no other captures, meaning a and f pawns promoted straight down.
If only white could retract f2xe3, the position would unlock, but he must wait until black unpromotes on f1 and retracts that P to f3.
The best bet is an N promotion on f1, but it just isn't fast enough. White's g pawn can uncap an N at several places, but white runs out of pawn retractions with black's P on f2 in all tries.
BigDoggProblem wrote:
daStrwbrry wrote:
#8676
Black is apparently missing 2P and 1 N. White made 3 captures.
White is missing LSB, which was captured on f1, and N, which must have been captured on the c-file. Black made no other captures, meaning a and f pawns promoted straight down.
If only white could retract f2xe3, the position would unlock, but he must wait until black unpromotes on f1 and retracts that P to f3.
The best bet is an N promotion on f1, but it just isn't fast enough. White's g pawn can uncap an N at several places, but white runs out of pawn retractions with black's P on f2 in all tries.
Nice work with both positions! The first position was meant to be slightly harder than the second, but you still provided brilliant and concise analysis for both.
The second position seems to be just another case of white running out of tempo by one move. However, it does feature some rather interesting knight geometry: a N on f1 takes 1 move to g3, 2 moves to g4, 3 moves to g5, and 4 moves to g6. Of course, the sooner white uncaptures the bN, the more pawn tempi available, but also more moves for the bN to retract to f1.
I don't have a proof, but wanted to see how close I could come. This almost works, but there's no way to have the bishops trade places.
You can retract moves and even free the white King, but the Bishops are still stuck in a "corridor" at the beginning of this sequence:
How does black retract the King away?
White can't retract the b3 pawn to b2 with the Bc1 not home, and can't retract the c3 pawn to c2 without losing the benefit of the b1 square, which could allow the Bishops to pass each other.
Yeah, that’s pretty much the important bits of this position covered; the narrow bishop corridor is what I wanted to showcase here. It isn’t much different from #8281 (which shares a lot of relevant retro-analysis with this position), but it is a little something to revive the thread.