Some remarks on graded nilpotent Lie algebras and the Toral Rank Conjecture

Abstract

If $\mathfrak{n}$ is a $\mathbb{Z}^d_+$-graded nilpotent finite dimensional Lie algebra over a field of characteristic zero, it is well known that $\dim H^{\ast }(\mathfrak{n})\geq L(p) $ where $p$ is the polynomial associated to the grading and $L(p)$ is the sum of the absolute values of the coefficients of $p$. From this result Deninger and Singhof derived the Toral Rank Conjecture (TRC) for 2-step nilpotent Lie algebras. An algebraic version of the TRC states that $\dim H^{\ast }(\mathfrak{n})\geq 2^{\dim (\mathfrak{z)}} $ for any finite dimensional Lie algebra $\mathfrak{n}$ with center $\mathfrak{z}$. The TRC is more that 25 years old and remains open even for $\mathbb{Z}^d_+$-graded 3-step nilpotent Lie algebras. Investigating to what extent the above bound for $\dim H^{\ast }(\mathfrak{n})$ could help to prove the TRC in this case, we considered the following two questions regarding a nilpotent Lie algebra $\mathfrak{n}$ with center $\mathfrak{z}$: (A) If $\mathfrak{n}$ admits a $\mathbb{Z}_+^d$-grading $\mathfrak{n}=\bigoplus_{\alpha\in\mathbb{Z}_+^d} \mathfrak{n}_\alpha$, such that its associated polynomial $p$ satisfies $L(p)>2^{\dim\mathfrak{z}}$, does it admit a grading $\mathfrak{n}=\mathfrak{n}'_{1}\oplus \mathfrak{n}'_{2}\oplus \dots\oplus \mathfrak{n}'_{k}$ such that its associated polynomial $p'$ satisfies $L(p')>2^{\dim\mathfrak{z}}$? (B) If $\mathfrak{n}$ is $r$-step nilpotent admitting a grading $\mathfrak{n}=\mathfrak{n}_{1}\oplus \mathfrak{n}_{2}\oplus \dots\oplus \mathfrak{n}_{k}$ such that its associated polynomial $p$ satisfies $L(p)>2^{\dim\mathfrak{z}}$, does it admit a grading $\mathfrak{n}=\mathfrak{n}'_{1}\oplus \mathfrak{n}'_{2}\oplus \dots\oplus \mathfrak{n}'_{r}$ such that its associated polynomial $p'$ satisfies $L(p')>2^{\dim\mathfrak{z}}$? In this paper we show that the answer to (A) is yes, but the answer to (B) is no.