Let $p(n)$ denote the number of partitions of a positive integer $n$. It is known that $\{p(n)\}\_{n>25}$ is log-concave.
Dietrich Burde said [in this MathOF post](https://mathoverflow.net/questions/138321/is-the-sequence-of-partition-numbers-log-concave) that property $PF\_3$ for partition function $p(n)$ semms to hold for all $n>221$. And he said that the question is also motivated from the study of Betti numbers for nilpotent Lie algebras, in particular filiform nilpotent Lie algebras.


But I try to find the relationship between Partition function and Betti numbers, but failed. 


**Need Help.**



What is the relationship between Partition function and Betti numbers for nilpotent Lie algebras?
The relationship is as follows. Denote by $\mathfrak{f}\_n$ the standard graded filiform nilpotent Lie algebra of dimension $n$. Then the following result is known.


*Theorem (Armstrong, Sigg 1996):* Let $L$ be a complex nilpotent Lie algebra having an abelian ideal of codimension $1$. Then the sequence of Betti numbers $(b\_i(L))\_{i\ge 1}$ is unimodal. In particular, the sequence $(b\_i(\mathfrak{f}\_n))\_{i\ge 1}$ is unimodal.


There are formulas for the Betti numbers of $\mathfrak{f}\_n$ in terms of restricted partitions. The $r$-th Betti number of $\mathfrak{f}\_{n+1}$ is given by
$$ b\_r(\mathfrak{f}\_{n+1})=P\_{r,n}+P\_{r-1,n}$$
for $1\le r \le n+1$, where $P\_{0,n}=1$ and
$$P\_{r,n}=\# \Bigl \{ (a\_1,\ldots ,a\_r)\in \mathbb{Z}^r \mid 1\le a\_1 < \dots < a\_r
\le n,\quad \sum\_{j=1}^r a\_j =\Bigl \lceil \frac{r(n+1)}{2} \Bigr \rceil
\Bigr \}.$$
An explicit formula is only known for small $r$:
\begin{align\*}
b\_1(\mathfrak{f}\_{n}) & = 2 \\
b\_2(\mathfrak{f}\_{n}) & =\left \lfloor \frac{n+1}{2} \right \rfloor ,\\
b\_3(\mathfrak{f}\_{n}) & = \left \lfloor \binom{\frac{n+1}{2}}{2}+\frac{1}{8}
\right \rfloor = \left \lfloor \frac{n^2}{8}\right \rfloor,\\
b\_4(\mathfrak{f}\_{n}) & = \left \lfloor \frac{4}{3}\binom{\frac{n+1}{2}}{3}+
\frac{4n+13}{36} \right \rfloor = \left \lfloor \frac{(n-1)^3+18}{36}\right \rfloor .\\
\end{align\*}


Computing these Betti numbers $b\_i$ for $n\le 50$ I found that they form a log-concave sequence, for $i\ge 2$. So the the following question came up:


**Question:** Is it true that the sequence of Betti numbers $(b\_i(\mathfrak{f}\_n))\_{i\ge i\_0}$ for some $i\_0>1$ is log-concave?


The first thing which one would like to use for a proof here is that the ordinary partition function itself is log-concave. Already this was not clear to me. Hence my question at [MO](https://mathoverflow.net/questions/138321/is-the-sequence-of-partition-numbers-log-concave).


