B-star-algebra

• (x + y)^* = x^* + y^* for all x, y in A

(the involution of the sum of x and y is equal to the sum of the involution of x with the involution of y)

• (λ x)^* = λ^* x^* for every λ in C and every x in A; here, λ^* stands for the complex conjugation of λ.

• (xy)^* = y^* x^* for all x, y in A

(the involution of the product of x and y is equal to the product of the involution of x with the involution of y)

• (x^*)^* = x for all x in A

(the involution of the involution of x is equal to x)

If the following property is also true, the algebra is actually a C^*-algebra:

• ||x x^*|| = ||x||² for all x in A.

(the norm of the product of x and the involution of x is equal to the norm of x squared )

See also: algebra.