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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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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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