# group cohomology

In this post we will talk about cohomology(from homological algebra), group cohomology and its applications.

Why should we study (co)homology? Of course there are historical motivations. Yet a modern point of view is to compensate for the non-exactness of a functor from a category(with good properties, for example, an abelian category) to another. So, one more fundamental question arises: why should we concern ourselves with exactness of a sequence? Perhaps the motivation for this comes from the fact that a splitting sequence of abelian groups implies that this sequence is exact. This is the most common necessary condition for a sequence to split.

Now let’s get down to some concrete things, we suppose that $\mathfrak{C}$ is an abelian category, one prototype example is the category of modules over a commutative ring with unit. We consider the associated ($\mathbb{Z}$-)graded category $\mathfrak{C}'$, in which objects are $(C_i,\delta_i)_{i\in\mathbb{Z}}$ where $\delta_i:C_i\rightarrow C_{i-1}$ is a morphism in $\mathfrak{C}$ such that $\delta_i\circ \delta_{i-1}=0$. So the first motivational question comes: why de we consider this kind of things? One motivation comes from differential manifolds. In the theory of differential manifolds, we have an important operator, the exterior differential operator, $d$, which acts on differential forms. Differential forms give a $\mathbb{Z}$(or $\mathbb{N}$)-graded real vector space, and $d\circ d=0$. This is an important property of $d$. And the category $\mathfrak{C}'$ can be seen as a generalization of this object.

Now a morphism from $(C_i,\delta_i),(D_i,\partial_i)$ is a sequence of maps $f_i:C_i\rightarrow D_i$ such that $\partial_i\circ f_i=f_{i-1}\circ \delta_i$. This definition is very reasonable.

For each such object $C=(C_i,\delta_i)$, we consider $H_n(C)=Ker(\delta_i)/Im(\delta_{i+1})$. This is of course a most fundamental concept in the theory of (co)homology. One straightforward motivation for these $H_n(C)$ is to compensate for the non-exactness of these maps $\delta_i$. If $f:C=(C_i,\delta_i)\rightarrow D=(D_i,\partial_i)$ is a morphism, then it is easy to verify that $f$ induces maps $H_n(f):H_n(C)\rightarrow H_n(D)$.

Now let’s consider another different question: given two maps $f,g:C=(C_i,\delta_i)\rightarrow D=(D_i,\partial_i)$, when will $H(f)=H(g)$? This is a rather interesting question. One sufficient condition comes from homotopy theory, which is, at first sight, completely different from homology theory. We say that two such maps $f,g$ are homotopy equivalent if there is a sequence of maps $h_n:C_n\rightarrow D_{n+1}$ such that $f_n-g_n=\partial_{n+1}\circ h_n+h_{n-1}\circ \delta_n$. It is very easy to show that if $f, g$ are homotopy equivalent, then they induce the same maps $H_n(f)=H_n(g),\forall n$. This is really a fundamental verification in homology theory. Recall the domains of $H_(f),H(g)$: they are $H_n(C)=Ker(\delta_n)/Im(\delta_{n+1})$. Thus for any $x\in H_n(C)$, we have that $f_n(x)-g_n(x)=\partial_{n+1}\circ h_n(x)+h_{n-1}\circ \delta_n(x)$. Note that $\delta_n(x)=0$, besides $\partial_{n+1}\circ h_n(x)\in Im(\partial_{n+1})$, thus $f_n(x)=g_n(x),\forall x\in H_n(C)$. It is also easy to show that homotopy equivalent is an equivalent relation. Note here why should we need the first term $\partial_{n+1}\circ h_n$? In fact these two terms can be seen from homotopy theory. That is what we are going to say in the next post.

# Clichés transmitted by professors of maths/physics undergraduates in France

The latest issue of Gazette des Mathématiciens, published by SMF, is out today.

Many interesting articles there, but I’d like to mention for now the one (pages 53 to 58) by Arnaud Pierrel, a PhD student in Sociology, who writes in the Parité column.

He studies students who are attending Classes Prépa Scientifiques (in short CPGE, that’s 2 years of very intensive undergraduate courses just after high school at the end of which students attempt various competitive exams to enter a variety of Schools, mainly in engineering but also in Math or Computer Science like the École Normale Supérieure in Paris, ÉNS).

One stunning statistic from Pierrel’s article, which underlines the sociological clichés that teachers are (subconsciously?) transmitting to their students,  is the following (I’ve cut some inessential details, translation below):

Dans le cadre de notre recherche sur les CPGE scientifiques, nous avons suivi une promotion…

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