Suppose we have a Banach algebra A without unit. We can always embed into an algebra with unity such that the elements in are of the form

We know that under the norm , is a Banach algebra and with the involution , ia a Banch *-algebra.

From the book, it is stated there that can be a Banach *-algebra under many norms. Can anyone give me any example of norms that can make a Banach *-algebra?