Journal of Algebra, Number Theory: Advances and Applications[1] C. Kassel, Quantum Groups, Graduate Texts in Mathematics,
Springer-Verlag, Berlin/ New York, 155 (1995).
[2] J. Kock, Elementary remarks on units in monoidal categories, Math.
Proc. Cambridge Philos. Soc. 144(1) (2008), 53-76.
[3] S. Mac Lane, Natural associativity and commutativity, Rice
University Studies 49(4) (1963), 28-46.
[4] N. T. Quang, Introduction to Ann-categories, Tap chi Toan hoc,
Hanoi 15 (1987), 14-24 [arXiv: 0702588 v2 [math.CT], (21 Feb
2007)].
[5] N. T. Quang, Enough strict Ann-categories (in Vietnamese), J.
Mathematics 20(1) (1992), 41-47.
[6] N. T. Quang, On Gr-functors between Gr-categories: Obstruction
theory for Gr-functors of the type 
arXiv: 0708.1348 v2 [math.CT], (2009).
[7] N. T. Quang and D. D. Hanh, Cohomological classification of
Ann-functors, East-West Journal of Mathematics 11(2) (2009),
195-210.
[8] N. Saavedra Rivano, Categories Tannkiennes, Lecture Notes in
Mathematics 265, Springer-Verlag, Berlin-Heidelberg-New York, 1972.
[9] Peter Schauenburg, Turning monoidal categories into strict ones,
New York J. Math. 7 (2001), 257-265.
