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arXiv 1306.1593math.COmath.RT
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根偏序集及其富反链

The root posets and their rich antichains

Claus Michael Ringel

摘要:

设$Δ$是秩$n\ge 2$的(连通)Dynkin图,$Φ_+ = Φ_+(Δ)$是对应的根偏序集(它由关于固定根基的所有正根组成). $Φ_+$的宽度为$n$。我们将证明$Φ_+$是“圆锥”的:它是$n$固体链的不相交并。$Φ_+$中丰富的反链是基数$n-1$的反链。众所周知,富反链的数目等于$Φ_+$的基数。$Φ_+$中的富反链集合$\mathcal R(Δ)$本身可以被认为是一个偏序集,它与$Φ_+$非常相似,但并不总是同构。我们将证明,总是存在一个独特的丰富的反链$A$,使任何丰富的反链包含在理想所产生的$A$。对于$Δ\neq \Bbb E_6$,$A$中的所有根都具有相同的长度,即$e_2$,其中$e_1 \le e_2 \le \dots \le e_n$是$Δ.$的指数。对于$Δ= \Bbb E_6$,反链$A$由四个长度为$e_2 = 4$的根和一个长度为$5$的根组成。

英文摘要:

Let $Δ$ be a (connected) Dynkin diagram of rank $n\ge 2$ and $Φ_+ = Φ_+(Δ)$ the corresponding root poset (it consists of all positive roots with respect to a fixed root basis). The width of $Φ_+$ is $n$. We will show that $Φ_+$ is "conical": it is the disjoint union of $n$ solid chains. The rich antichains in $Φ_+$ are the antichains of cardinality $n-1$. It is well known that the number of rich antichains is equal to the cardinality of $Φ_+$. The set $\mathcal R(Δ)$ of rich antichains in $Φ_+$ can itself be considered as a poset which is quite similar, but not always isomorphic, to $Φ_+$. We will show that there always exists a unique rich antichain $A$ such that any rich antichain is contained in the ideal generated by $A$. For $Δ\neq \Bbb E_6$ all roots in $A$ have the same length, namely $e_2$, where $e_1 \le e_2 \le \dots \le e_n$ are the exponents of $Δ.$ For $Δ= \Bbb E_6$, the antichain $A$ consists of four roots of length $e_2 = 4$ and one root of length $5$.

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