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In hindsight, though, he now thinks that oversight was a brilliant mistake. The fact that no one controlled sudoku’s intellectual property rights let the game’s popularity grow unfettered, Mr. Kaji says. Nikoli does not plan to trademark other new games, either, in hopes this will also help them take off. “This openness is more in keeping with Nikoli’s open culture,” said Mr. Kaji, who sat on a sofa in his Tokyo office among pillows adorned with printed faces of racehorses. “We’re prolific because we do it for the love of games, not for the money.”