Excellent problem! I didn't attempt to solve it because it seemed too hard but in the end it would have been doable. There is a mystery. Why is Pb6 on the board? That could only be because of a white dual but I can't find one. Not in 23 moves that is!
Mate In 23-Double Maximummer!
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A "maximummer" is a chess problem in which Black plays the geometrically longest move. If in check, a side must play the longest move that gets them out of check. See more details here-http://chess-problems-gr.blogspot.com/2008/05/maximummer.html
-and here- http://figjamchess.blogspot.com/2013/08/the-most-geometrically-awesome-chess.html
But some problems are a "double maximummer," that is both sids make the longest possible move. Given this information, can you solve this double maximummer? It's White to move and give mate in 23 moves.
Arnold Beine, Die Schwalbe 2002