# More on Periodicity and Duality associated with Jordan partitions

15 Jul 2019  ·  Barry Michael J. J. ·

Let $J_r$ denote a full $r \times r$ Jordan block matrix with eigenvalue $1$ over a field $F$ of characteristic $p$. For positive integers $r$ and $s$ with $r \leq s$, the Jordan canonical form of the $r s \times r s$ matrix $J_{r} \otimes J_{s}$ has the form $J_{\lambda_1} \oplus J_{\lambda_2} \oplus \dots \oplus J_{\lambda_{r}}$ where $\lambda_1 \geq \lambda_2 \geq \dots \geq \lambda_{r}>0$. This decomposition determines a partition $\lambda(r,s,p)=(\lambda_1,\lambda_2,\dots, \lambda_{r})$ of $r s$, known as the \textbf{Jordan partition}, but the values of the parts depend on $r$, $s$, and $p$. Write $(\lambda_1,\lambda_2,\dots, \lambda_{r})=(\overbrace{\mu_1,\dots,\mu_1}^{m_1},\overbrace{\mu_2,\dots,\mu_2}^{m_2},\dots, \overbrace{\mu_k,\dots,\mu_k}^{m_k}) =(m_1 \cdot \mu_1, \dots,m_k \cdot \mu_k),$ where $\mu_1>\mu_2>\dots>\mu_k>0$, and denote the composition $(m_1,\dots,m_k)$ of $r$ by $c(r,s,p)$. A recent result of Glasby, Praeger, and Xia in \cite{GPX} implies that if $r \leq p^\beta$, $c(r,s,p)$ is periodic in the second variable $s$ with period length $p^\beta$ and exhibits a reflection property within that period. We determine the least period length and we exhibit new partial subperiodic and partial subreflective behavior.

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Representation Theory