This is an additional question to [that question](https://mathoverflow.net/questions/255556/nilpotent-infinite-binary-matrices), inspired by Leonid Petrov.


Let $\text{Mat}(\mathbb{N},\{0,1\})$ be the set of all maps $A:\mathbb{N}\times\mathbb{N}\to \{0,1\}$. We define a matrix multiplication for $A, B\in \text{Mat}(\mathbb{N},\{0,1\}$) and $m,n\in\mathbb{N}$ by $$(A B)(m, n) = 0 \text{ if } \sum\_{i=1}^\infty A(m, i)B(i, n) < \infty \text{ and } (AB)(m, n) = 1 \text{ otherwise.}$$


Is this kind of matrix multiplication associative?



Is this operation on infinite matrices associative?
No, even $(A^2)A\ne A(A^2)$ in general. We consider $A$ as an incidence matrix of an infinite graph (with possible loops, and directed, but in our example it is loop-less and undirected). Consider vertices 1, 2 and join them but infinitely many paths $1x\_{ij}y\_j2$, where $i,j=1,2,\dots$, and all vertices $y\_j,x\_{ij}$ are distinct. Then $(A^2)\_{1y\_j}=1$ for all $j$, hence $((A^2)A)\_{12}=1$, but $(A^2)\_{k2}=1$ only for $k=2$, thus $(A(A^2))\_{12}=0$.


