Mathematics Stack Exchange is a question and answer site for people studying math at any level and professionals in related fields. It's 100% free, no registration required.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

For any $x,y \in S$ we have $$f(y)=f(x) + \nabla f(x)^T(y-x) + \frac{1}{2}(y-x)^T\nabla^2f(z)(y-x) $$

for some $z$ on the line segment $[x,y].$

I don't see why this should be true, other than that it 'feels' right. Is there a simple proof? Also, I'm not sure if the title is most appropriate, so feel free to edit it.

share|cite|improve this question
I'm assuming of course that the function is sufficiently smooth for second derivative to exist and be continuous. – Squirtle Jul 10 '13 at 0:05
This is just Taylor's theorem, with the remainder in a certain form. You can derive Taylor's theorem easily using integration by parts. Then use the mean value theorem for integration to get the remainder in the desired form. See – wj32 Jul 10 '13 at 0:10
Cool... thanks! – Squirtle Jul 10 '13 at 20:10

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Browse other questions tagged or ask your own question.