# Tag Info

Accepted

### Number of self-inverses in a group

An element of order $2$ in a group is often called an involution. In geometric groups reflections are involutions, as are half turns and central involutions, for example. Involutions are normally ...
• 97.1k
Accepted

### Is there a name for the opposite of (vector) projection?

This is called vector rejection. A common concept in Geometric algebra.
• 9,067

### Inverse Trig Identity Proof under different conditions

The identity in your book only holds for when $x > 0, y > 0$: else $\arctan x + \arctan y =$ $\arctan \left(\frac{x+y}{1-xy} \right) - \pi$ instead where $x < 0$ and $y < 0$ (the two cases ...
• 16k