Reduction of the search space to find perfect play of 6x6 board Othello

In 1993, mathematician Feinstein found out perfect play on 6x6 board Othello gives 16-20 loss for the first player by using computer. He reported on the Web that it took two weeks to search forty billion positions in order to obtain the result. In our previous papers, we confirmed the perfect play he found is correct. And we also found another perfect play different from the one he found to search 884 billion positions. In order to search efficiently, we attempted to reduce the search space to find some perfect play. In this paper, we introduce some techniques to solve 6x6 board Othello by searching about nine billion positions.