Wry neck, or torticollis, is a painfully twisted and tilted neck. The top of the head generally tilts to one side while the chin tilts to the other side. This condition can be congenital (present at birth) or acquired. It can also be the result of damage to the neck muscles or blood supply. Wry neck sometimes goes away without treatment.
In the quaternions, 0 is the additive identity. In the ring of functions from R to R, the function mapping every number to 0 is the additive identity. In the additive group of vectors in Rn, the origin or zero vector is the additive identity.
For addition, the identity is zero, because adding zero to anything doesn't change anything. The inverse is the additive inverse: it's the same number, but with the opposite sign. For instance, suppose your number is â6, and you're adding. The identity is zero, and the inverse is 6, because â6 + 6 = 0.