Either you got the description wrong or I'm especially rusty - in that case, f''... | Hacker News

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comex on July 10, 2013 | parent | context | favorite | on: Math, Science Popular Until Students Realize They’...

Either you got the description wrong or I'm especially rusty - in that case, f''(x) = e^(-x), which is positive even when x is negative, so it doesn't always have the same sign.

wging on July 10, 2013 [–]

Oh, crap, you're right. I read that as f''(x) must have the same sign as f(x).

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