优点：输出在 (−1,1)，比 sigmoid 居中，对梯度更友好
遂想起王安石所言：“贫者不得行礼，贵者得行无礼。”——意思是，普通人有心无力，对天下风俗礼仪，即便想认真隆重，也起不了什么作用；而有权有钱人有条件，却恰恰不带头淳化隆厚风习礼俗，反而争相毁弃文雅、蔑视礼仪，甚至不以粗鄙为耻，反以为荣。这是很危险的，王荆公将此列为末世风俗。
。业内人士推荐快连下载安装作为进阶阅读
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Quadtrees are everywhere spatial data exists. Mapping services use quadtree-like tile pyramids to serve map tiles at different zoom levels (Bing's quadkey system, for example, addresses tiles as base-4 paths). Game engines use them for collision detection and visibility culling. Geographic information systems use spatial indexes to store and query spatial datasets. PostGIS uses GiST indexes (R-tree-style) for spatial queries on geometries, while PostgreSQL's core supports quadtree-like SP-GiST indexes for certain data types like points.