マイナス・ガンマの混乱

最近オプションのガンマを考えています。プットのロングはガンマがマイナスになるはずですがプラスです。As the underlying's value rises for a put, the put's value declines and the delta goes to zero. As the underlying's value falls, the put gains in value and the delta goes to negative one(not one). Thus, since the delta's range is [-1,0], the gamma is in fact always positive for the long put.