Victor Korchnoi has always considered 13 his lucky number, and yesterday he decided to test that theory in the 13th game of his world championship chess match with Anatoly Karpov. Korchnoi sacrificed the exchange (a rook for knight and pawn) near the end of yesterday's session, and two moves later the game was adjourned in the kind of complex, dynamic, unbalanced position he finds specially congenial.

Although champion Karpov has a slight material advantage in the adjourned position. Korchnoi enjoys an initiative and seems to have strong prospects of capturing his opponent's isolated QBP. If he manages to eliminate that pawn while keeping his own pawn center intact, he will have two linked passed pawns in the center, which could be a winning advantage. In this case, Karpov's hopes for holding a draw will rest largely on the fact that the two bishops remaining on the board occupy squares of opposite color.

Korchnoi could undoubtedly eliminate the black bishop at the cost of a knight and pawn by playing 41. N-K5, a move that Karpov's 40 . . . B-N2 war clearly designed to inhibit. An immediate 41. P-K4 seems premature and would probably lead to an exchange of queens after 41 . . . Q-K3.

Although the threatened pawn is attacked twice and defended only once, Korchnoi cannot play 41. BxP, as this would lose a piece: 41. BxP, RxB; 42. QxR, QxN. Most probable is a move that would increase pressure on the weak pawn, either 41. R-R6 or 41.N-N4. The knight move would allow Karpov to play 41. ... Q-N8ch, but it is hard to see any long-term advantage for the champion resulting from this option. Korchnoi is apparently fighting to win, while his opponent is struggling for a draw.

The game will be resumed today, after both sides spend the night intensively analyzing the possibilities of the position.