**Bibliography:**

[1] Andruskiewitsch and Graña, From racks to pointed Hopf algebras, Adv. Math. 178 (2003) 177-243

]]>In this talk we assign a free crossed complex to a cover $\mathcal{U}$ of the topological space $X$, so we get the notion of nonabelian cohomology.

We finish our talk by introducing a long exact sequence for nonabelian cohomology.

**Bibliography:**

[1] R. Brown , P. Higgins and R. Sivera. Nonabelian algebraic topology. European Mathematical Society, 2010

[2] T. Nikolaus and K. Waldorf. Lifting problems and transgression for non-abelian gerbes, Advances in Mathematics 242, pp. 50–79, 2013

[3] L. Breen. Bitorseurs et cohomologie non abélienne, The Grothendieck Festschrift, pp. 401–476, 2007.

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