Why use transpose of nabla in gradient descent For gradient descent we have the formula:
$f(x_{k}+d_{k})\approx f(x_{k}) + \nabla f(x_{k})^T  d_{k}  $
What I don't understand is, why we use the transpose of nabla and not just nabla.

Note that if f is scalar, you need an inner product to make a scalar on the right hand side.  Generally nabla will be a vector
