Maybe this is too technical and elementary, but I cannot make up my mind, nor find a reference.


The situation is the following: let $X$ be a double cochain (right half-plane) complex of abelian groups and let 


$$
(\mathbf{Tot}^{\prod} X)^n = \prod\_{p+q = n}X^{pq}
$$


denote its total-product complex. The first filtration on $X$,


$$
{}\_I F^s(X) = 
\begin{cases}
X^{pq} & \ \text{if} \ p \geq s , \\\
0 & \ \text{otherwise}
\end{cases}
$$


gives you the filtration on $\mathbf{Tot}^\prod X$:


$$
(F^s \mathbf{Tot}^\prod X)^n = \prod\_{p+q=n , p\geq s} X^{pq} 
$$


and you have, as with any filtered differential complex $(A, F, d)$, an induced filtration in cohomology:


$$
F^pHA = \mathbf{im} (HF^pA \longrightarrow HA) \ .
$$


My question is the following: is the filtration induced by ${}\_I F$ on $H(\mathbf{Tot}^\prod X)$ Hausdorff? That is, 


$$
\bigcap\_p F^pH(\mathbf{Tot}^\prod X ) = 0 \ ?
$$


I couldn't find an answer in the literature. Weibel's book says that in this situation the spectral sequence arising from the first filtration is "convergent". Unfortunately, for Weibel this only means that you have an isomorphism $E\_0 HA = E\_\infty A$. Cartan-Eilenberg "Homological Algebra" doesn't work with the total-product complex, but with the total-sum one:


$$
(\mathbf{Tot}^{\bigoplus} X)^n = \bigoplus\_{p+q = n}X^{pq}
$$


For this one, I think, the answer is "yes": if I had some cohomology class


$$
[x] \in \bigcap\_p F^pH^n(\mathbf{Tot}^\bigoplus X ) \quad \Longleftrightarrow \quad [x] \in F^pH^n(\mathbf{Tot}^\bigoplus X ) \ \text{for all} \ p
$$


then I could find representatives for $[x]$ like


$$
(0, \stackrel{p-1}{\dots}, 0, x^{p,n-q}, x^{p+1, n-q-1}, \dots )
$$


for all $p \geq 0$. Since there is only a finite number of $x^{pq} \neq 0$ for each element of $(\mathbf{Tot}^\bigoplus X)^n$, in a finite number of steps, I can be sure that I can find a representative for $[x]$ which is zero, so $[x]=0$.


But this reasoning doesn't work with $\mathbf{Tot}^\prod X$: you can only state with certainty that, for every $p$ there is some $x\_p \in F^p$ and $b\_p$ such that 


$$
x - x\_p = db\_p \quad \Longleftrightarrow \quad x- db\_p \in F^p \ .
$$


These equations have a nice interpretation: if you topologize $\mathbf{Tot}^\prod X$ taking as basic open sets $x + F^p$ for all $x \in \mathbf{Tot}^\prod X$ and $p$, they read:


$$
(db\_p) \longrightarrow x \ .
$$


That is, $x$ is a limit of coboundaries. But, unless the filtration is finite, this doesn't imply that $x$ itself *is* a coboundary, does it?


So I could ask my question this way: in this situation, is the set of coboundaries closed?



Is the first filtration Hausdorff?
No, this filtration is not necessarily Hausdorff. The problem is connected to nonexactneess of the inverse limit functor. Here is a family of examples to illustrate the basic issue.


Suppose $\cdots \to A\_2 \to A\_1 \to A\_0$ is an inverse system of abelian groups. Define a complex by
$$
X^{p,-p} = X^{p+1,-p} = A\_p
$$
for all natural numbers $p$, with $X^{p,q} = 0$ otherwise. The horizontal differentials $X^{p,-p} \to X^{p+1,-p}$ are the identity maps $A\_p \to A\_p$, and the vertical differentials $X^{p,-p} \to X^{p,1-p}$ are the maps $A\_{p} \to A\_{p-1}$ from the inverse system for $p > 0$.


The total complex has only two nonzero groups in degrees 0 and 1:
$$
\prod A\_p \to \prod A\_p
$$
and the indicated map is the map Milnor introduced whose kernel is $\lim A\_p$ and whose cokernel is $\lim^1 A\_p$. (Possibly up to sign, depending on what your conventions for a double complex are.) Therefore
$$
H^0 Tot^{\Pi} X = \lim A\_p,\ H^1 Tot^\Pi X = {\lim}^1 A\_p
$$
However, the spectral sequence associated to this complex has the amusing behavior that the "diagonal" entries $E\_r^{p,-p}$ busily spawn differentials that erase everything on the "superdiagonal" $E\_r^{p+1,-p}$. Therefore, the entirety of $H^1(Tot^\Pi X)$ is in the intersection of the images of the filtrations.


(As a mostly nonmathematical aside, $\lim^1$ is the bane of many an argument.)


If you're interested in convergence properties, I'd suggest Boardman's thesis, mentioned in [this answer](https://mathoverflow.net/questions/32762/convergence-of-right-half-plane-spectral-sequence-bounded-on-the-right/32769#32769) of Tilman's.


