符号的辨析

Ο，读音：big-oh；表示渐进上界，小于等于。
ο，读音：small-oh；表示上界，小于。
Ω，读音：big omega、欧米伽；表示渐进下界，大于等于。
ω，读音：small omega；表示下界，大于。
Θ，读音：theta、西塔；既是上界也是下界，称为确界，等于。

Ο是渐进上界，Ω是渐进下界。Θ需同时满足大Ο和Ω，故称为确界。Ο极其有用，因为它表示了最差性能。

$o(3n^2+52)=O(n^2)$

以前就只知道要忽略常数

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