# Gradient of vector???

I have read the book about machine learning. And it use gradient, but problem is D is orthogonal matrix and c is vector. I don't understand way that it gradient to c^Tc become 2c. What knowledge do I need to learn to understand it?

• Gradient expands whatever you have before to make a partial derivative with respect to everything you consider to be a variable. Increasing the number of indexes by one with one new specific number in that index for that variable.. – mathreadler Oct 28 '17 at 14:59
• It's just the gradient of $c_1^2+\dots+c_n^2$. – Hans Lundmark Oct 28 '17 at 15:07
• I understood, thanks you. – QChí Nguyễn Oct 28 '17 at 15:14