**Properties**

As the last example shows, it is possible for (*S*, ∗) to have several left identities. In fact, every element can be a left identity. Similarly, there can be several right identities. But if there is both a right identity and a left identity, then they are equal and there is just a single two-sided identity. To see this, note that if `l` is a left identity and `r` is a right identity then *l* = *l* ∗ *r* = *r*. In particular, there can never be more than one two-sided identity. If there were two, `e` and `f`, then *e* ∗ *f* would have to be equal to both `e` and `f`.

It is also quite possible for (*S*, ∗) to have *no* identity element. The most common example of this is the cross product of vectors. The absence of an identity element is related to the fact that the direction of any nonzero cross product is always orthogonal to any element multiplied – so that it is not possible to obtain a non-zero vector in the same direction as the original. Another example would be the additive semigroup of positive natural numbers.

Read more about this topic: Identity Element

### Famous quotes containing the word properties:

“A drop of water has the *properties* of the sea, but cannot exhibit a storm. There is beauty of a concert, as well as of a flute; strength of a host, as well as of a hero.”

—Ralph Waldo Emerson (1803–1882)

“The reason why men enter into society, is the preservation of their property; and the end why they choose and authorize a legislative, is, that there may be laws made, and rules set, as guards and fences to the *properties* of all the members of the society: to limit the power, and moderate the dominion, of every part and member of the society.”

—John Locke (1632–1704)