Illegal Position Contest!

hint: black's position is legal

but white's is not

Correction: Black's position is illegal, but White's is not.

Can anyone prove that my position from post #239 is illegal? For convenience, I can transfer the diagram onto the current page:

oops, but thanks for the correction and proof!

I have no idea.

No rest for the weary! Who can prove this one is illegal?

ouch...

Uh....How did the a3 rook get there?

Rh3,Rh1,Ra1,Ra3

The Knight on a1 would not have been able to get there because of the pawns.

The king on h2. That simple.

Like this:

Are you sure about that?  

Alright, let's try this:

The only way the black king could have gotten to h2 is via a3 due to the position of the white pawns. The pawn b3 can originally be from a2 or b2.

1) If it comes from b2, it must have been there already when the king entered. As a consequence, the knight a1 must have been there as well, as it could not have gotten there after b3.

Also the white rook must have been inside the pawn formation already , as it could not have entered after b3 via the a file.

What makes the position illegal in this case is that the white rook and the black king could not have gotten past each other without an illegal check (e.g. Rh1, Kh2-g1?? or Ra2 Kb1-b2??). Ra1 Kc1-b2 is not possible because of the knight a1.

2) If the pawn b3 comes from a2, the pawn b3 and a4 must have captured. Neither the bishop c8 (which never moved) nor the rook a8 (which didn't escape the 8th row because of the bishop f8 and because the pawn b7 must have been there until the knight came to a8) could have been captured. Therefore, not enough pieces are available for two captures, as only three black pieces are missing. Hence, the position is illegal.

@DerfieseSchlemiel: Very good!   Even though there are a few slight corrections to be made, none have a serious impact on the soundness of your reasoning, so I consider this one solved. Bravo!

I'll still address the microscopic errors, though:

• "Also the White rook must have been inside the pawn formation already, as it could not have entered after b3 via the a file."  Technically, the rook could still be outside the pawn chain at that point, and later enter via the h-file. Of course, the same "traffic issue" you described would remain (the king and rook cannot get past each other), so this small detail is not particularly relevant.
• "(e.g. Rh1, Kh2-g1?? or Ra2 Kb1-b2??)"  In these attempts, the king and rook have already gotten past each other, so they're not the best examples. Better would be Rh1 Kg1-h2?? or Rb1 Kb2-c1??
• "...nor the rook a8 (which didn't escape the 8th row because of the bishop f8 and because the pawn b7 must have been there until the knight came to a8)..."  The logic here is accurate, but a tiny glitch in the explanation - I think you meant to say, "...and because the pawn b6 must have been on a7 until the knight came to a8".

But all in all, it was a job well done!