Consider a generic nontrivial $d$-cocycle $\omega\_d^G \in H^d(G,U(1))$ in the cohomology group of a group $G$ with $U(1)=\mathbb{R}/\mathbb{Z}$ coefficient. In otherwords, here the $d$-cocycle $\omega\_d^G$ is a complex $U(1)=\mathbb{R}/\mathbb{Z}$ function with the norm $|\omega\_d^G|=1$ but with a $U(1)$ complex phase satisfying the cocycle condition $ \delta\omega\_d^G=1$.



> 
> question: We like to ask **whether there always exists some Abelian group $N$ as a normal subgroup** of some bigger group $J$, such that $G$ is the quotient group
>  $$
> \frac{J}{N}=G
> $$
>  and **such that we can always trivialize (or split) the $d$-cocycle $\omega\_d$ of $G$ into $d$-coboundary if we lift $G$ into a larger group $J$**? Given that we know the group homomorphism $r$:
>  $$J \overset{r}{\rightarrow} G.$$
>  Namely,
>  $$
> \omega\_d^G(g\_i,\dots)=\omega\_d^G(r(j\_i),\dots)=
> \delta \beta\_{d-1}^J(j\_i,\dots). 
> $$
>  with $g \in G, j \in J$.
> 
> 
> 


You are welcome to comment or answer the partial case, for example, when $d=2$, $d=3$ or $d=4$, and when $G$ is a finite group. Partial comments or answers are still welcome!



$G$ cocycle split to a coboundary in $J$, via a group extension
In case d=1, the answer is always negative: 1-cocycles are homomorphisms, 1-coboundaries are always trivial, and inflation is injective. 
If you do not restrict yourself to the case where $N$ is abelian, the answer is positive: take a short exact sequence $1\to R\to F\to G\to 1$ where $F$ is free. For the abelian case we proceed as follows: you also have the short exact sequence $1\to R\_{ab}\to F/[R,R]\to G\to 1$ and you have a morphism of short exact sequences from the first one to the second one. This also gives you a morphism on the corresponding Lyndon-Hochschild-Serre spectral sequences. In particular we have the following commutative diagram: $\require{AMScd}$\begin{CD}H^{d-2}(G,H^1(R\_{ab}, U(1)))@> >> H^d(G,U(1))\\ @V V V @V V V \\ H^{d-2}(G,H^1(R,U(1)))@> >> H^d(G,U(1))\end{CD}
For $d>2$ the lower horizontal morphism is an isomorphism since the spectral sequence converges to the cohomology of F, which is trivial. The right vertical arrow is the identity, and the left vertical arrow is an isomorphism, since all homomorphisms from $R$ to $U(1)$ factor through $R\_{ab}$. But this implies that also the upper horizontal homomorphism is an isomorphism, and that the inflation from $G$ to $F/[R,R]$ is trivial. So this extension trivializes all cocycles at the same time. For $d=2$ we can similarly show that the upper horizontal morphism is surjective, and this is enough for the vanishing of the second cohomology. Another option: a two cocycle will give rise to an extension $$1\to U(1)\to J\to G\to 1.$$
The inflation of the cocycle to $J$ will be zero.


