Vector identities

This article lists a few helpful mathematical identities which are useful in vector algebra.

Triple Products

$\vec{A} \times (\vec{B} \times \vec{C}) = \vec{B}(\vec{A} \cdot \vec{C}) - \vec{C}(\vec{A} \cdot \vec{B})$
$\vec{A}\cdot(\vec{B}\times \vec{C}) = \vec{B}\cdot(\vec{C}\times \vec{A}) = \vec{C}\cdot(\vec{A}\times \vec{B})$

$\vec{\nabla} (fg) = f(\vec{\nabla}g) + g(\vec{\nabla} f)$
$\vec{\nabla}(\vec{A} \cdot \vec{B}) = \vec{A} \times (\vec{\nabla} \times \vec{B})+\vec{B} \times (\vec{\nabla} \times \vec{A})+(\vec{A} \cdot \vec{\nabla})\vec{B}+(\vec{B} \cdot \vec{\nabla})\vec{A}$
$\vec{\nabla} \cdot (f\vec{A})=f(\vec{\nabla} \cdot \vec{A})+\vec{A} \cdot (\vec{\nabla} f)$
$\vec{\nabla} \cdot (\vec{A} \times \vec{B})=\vec{B} \cdot (\vec{\nabla} \times \vec{A})-\vec{A} \cdot (\vec{\nabla} \times \vec{B})$
$\nabla\times (\vec{A}\times\vec{B})= (\vec{B}\cdot\nabla) \vec{A}-(\vec{A}\cdot\nabla)\vec{B} + \vec{A} (\nabla\cdot\vec{B}) - \vec{B}(\nabla\cdot\vec{A})$
$\nabla\times (f\vec{A})=f(\nabla\times\vec{A})-\vec{A}\times(\nabla f)$

$\vec{\nabla} \cdot \left( f \vec{\nabla} f \right) = f \vec{\nabla} \cdot \left( \vec{\nabla} f \right) + \left( \vec{\nabla} f \cdot \vec{\nabla} f \right) = f \nabla^2 f + \left( \vec{\nabla} f \right)^2$
therefore
$f \nabla^2 f = \vec{\nabla} \cdot \left( f \vec{\nabla} f \right) - \left( \vec{\nabla} f \right)^2$