csmath: Karpov's
18. Bb4
is a new move in this position which is a natural move. Before it has been played only 18. Bc7.
Peletier's first response was good (exchange knight for bishop) but his second response (19. ...f6) is bad. Clearly he wants a draw. It is uncomfortable to play against pawn majority on the kingside but he is making this even worse with going from 4 vs. 2 to 3 vs 1.
On the top of that destroying his own connected pair:
26. ...Be8?
is a serious positional error (against Karpov!) because Karpov's simple and natural responses lead to position after 30 moves with three bad indefensible black pawns. This is strategically lost position though black is not completely lost yet in a practical game. To make sure he loses (!) black plays completely anti-positional move
32. ...Rd7?
[32. ...h5 is his only chance to try to break white's pawn majority and to exchange bishops. The rook ending pawn down is bleak but black can put some serious resistance.]
that leads to rook exchange!
Now this is a note to all amateurs - never ever exchange rook going into same-color-bishop ending or a pawn ending down a pawn because you really have no hope to save that! A GM like Pelettier should definitely know that.
Karpov cleverly sets up wholesale exchange and with totally lost pawn ending in sight Pelletier resigns.
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This is the best game of Karpov here but not without a help of a distressed opponent.