# monoid v group

• Feb 4th 2011, 06:26 AM
monoid v group
Is a monoid the same thing as a group but without the requirement for elements to have an inverse element?
Thanks!
• Feb 4th 2011, 07:36 AM
tonio
Quote:

Is a monoid the same thing as a group but without the requirement for elements to have an inverse element?
Thanks!

Yes...at least according to the standard definition I know.

Tonio
• Feb 4th 2011, 01:26 PM
DrSteve
Essentially you're correct, but I wouldn't use the expression "same thing." Every group is a monoid, but not every monoid is a group. The integers under multiplication is an example of an abelian monoid which isn't a group (as are the rationals and reals).

A monoid has less structure than a group, so more structures are monoids, but monoids (that aren't groups) don't behave as nicely as groups.
• Feb 7th 2011, 12:49 AM
Swlabr
Quote:

Originally Posted by DrSteve
Essentially you're correct, but I wouldn't use the expression "same thing." Every group is a monoid, but not every monoid is a group. The integers under multiplication is an example of an abelian monoid which isn't a group (as are the rationals and reals).

A monoid has less structure than a group, so more structures are monoids, but monoids (that aren't groups) don't behave as nicely as groups.

Yes, but only groups can be abelian. Monoids are just commutative...

Another useful example of a monoid is the set of all (partial) functions from A to A. There is always an identity function, and some functions are bijections so have inverses. In general though, your functions will not have inverses. However, injections and surjections will have left or right inverses.