April 2020
Intermediate to advanced
344 pages
8h 22m
Japanese
Content preview from Pythonではじめる教師なし学習 ―機械学習の可能性を広げるラベルなしデータの利用
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20
1
章 機械学習エコシステムにおける教師なし学習の立ち位置
に改善される。
このような更新は計算量が大きいうえ、その過程で
2
つの大きな問題が起こりうる。
1
つはエラー関
数の勾配が非常に小さくなってしまうことだ。逆伝播（
backpropagation
）はこれらの小さい重みを掛
け合わせることで行われるため、ネットワークの重みの更新が非常に遅くなったり、完全に止まって
しまったりして、適切に学習ができなくなってしまう
＊
1
。この問題は勾配消失問題（
vanishing gradient
problem
）と 呼 ば れ る 。
もう
1
つの問題は、逆に誤差関数の勾配が非常に大きくなってしまうことだ。逆伝播の際に、ネット
ワークの重みの変化が大きくなってしまい、ネットワークの学習が非常に不安定になる。この問題は勾
配爆発問題（
exploding gradient problem
）と 呼 ば れ る 。
1.6.4.1
教師なし事前学習
非常に深い多層ニューラルネットワークを学習させることは困難だったので、複数のステージに分
けてニューラルネットワークを学習させる方法が考え出された。各ステージでは比較的浅いニューラル
ネットワークを学習させ、
1
つの浅いニューラルネットワークの出力を別の浅いニューラルネットワー
クの入力とする。このパイプラインの最初のニューラルネットワークでは教師なし学習を行い、後段の
ニューラルネットワークでは教師あり学習を行うのが一般的だ。
この教師なし学習の部分は貪欲層別教師なし事前学習（
greedy ...
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O’Reilly covers everything we've got, with content to help us build a world-class technology community, upgrade the capabilities and competencies of our teams, and improve overall team performance as well as their engagement.Julian F.
I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.