The Cuntz Algebra Q_N and C*-Algebras of Product Systems
  We consider a product system over the multiplicative semigroup N^x of Hilbert bimodules which is implicit in work of S. Yamashita and of the second named author. We prove directly, using universal properties, that the associated Nica-Toeplitz algebra is an extension of the C^*-algebra Q_N introduced recently by Cuntz. 
Jeong Hee Hong, Nadia S. Larsen, Wojciech Szymanski
math.OA