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**Hannibal lecter****Member**- Registered: 2016-02-11
- Posts: 225
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A line is neither concave up nor concave down.

what is that line, can I see a picture for it

I know the meaning of concave up and concave down.

but not others

Wisdom is a tree which grows in the heart and fruits on the tongue

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**George,Y****Member**- Registered: 2006-03-12
- Posts: 1,332

a straight line would satisfy these criteria

if concavity = second derivative of the single variable function

**X'(y-Xβ)=0**

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Definitions of convex and concave functions (Wikipedia):

︶ Convex (= convex down = concave up) function

︵ Concave (= convex up = concave down) function

According to the definitions above, we have:

1. All non-vertical lines (y = ax + b) are both concave up and concave down;

2. A vertical line (x = c) is neither concave up nor concave down.

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