※ 引述《elongation (no pain no gain)》之銘言：
: 請問有人懂微分幾何嗎？
: 想請問曲面的主曲率是指什麼？
: 主曲率所對應的主方向又是指什麼？
: 還有有人熟包絡理論和共軛理論的嗎？
: 我這裡有些問題想請教一下。
: 感謝。
假如你的主曲率和主方向指得是 principal curvature 跟 principal direction 的話，
do Carmo 書裡的定義如下
Let S be a regular surface with orientation N. For each p belong to S ,there
is a normal basis ｛e1,e2｝ of Tp(S) , DNp(e1)=-k1e1, DNp(e2)=-k2e2, k1≧k2
are the max and min of the second fundamental form restricted to S^1 of Tp(S).
k1,k2 are called the principal curvatures of S at p and the direction given
by e1 and e2 are called the principal directions.
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