$F_0$ counting (or estimating distinct elements, or "cardinality estimation") is very useful. Example: when you're doing profiling at the router level, you often want to estimate functions of distinct IP addresses, and since you can't just maintain counters for each possible address, $F_0$ counting turns out to be quite useful. $F_1$ counting, or ...


This is not an answer but a method which I believe would leave to an improved lower bound. Let us cut the problem after $a$ letters are read. Denote the family of $a$ element sets of $[n]$ by $\mathcal A$ and the family of $b=k-a$ element sets of $[n]$ by $\mathcal B$. Denote the states that can be reach after reading the elements of $A$ (in any order) by $...


Gan et al. address this in Moment-Based Quantile Sketches for Efficient High Cardinality Aggregation Queries. The answers are rather nuanced.


The definition says that every edge that exists has to go from some $V_i$ to $V_{i+1}$. It doesn't say that every possible edge from $V_i$ to $V_{i+1}$ has to be there. For example, $V_1=\{a,b\}$, $V_2=\{c,d\}$ with edges $ac$ and $bd$ gives a disconnected graph.

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