Representations of dimensions $(p^n\pm 1)/2$ of the symplectic group of degree $2n$ over a field of characteristic $p$

Abstract

The irreducible representations $\phi_n^1$ and $\phi_n^2$ of the symplectic group $G_n=Sp_{2n}(P)$ over an algebraically closednfield $P$ of characteristic $p>2$ with highest weights $\omega_{n-1}+\frac{p-3}{2}\omega_n$ and $\frac{p-1}{2}\omega_n$, respectively, are investigated. It is proved that the dimension of $\phi_n^i$ ($i=1,2$) is equal to $(p^n+(-1)^i )/2$, all weight multiplicities of these representations are equal to $1$, their restrictions to the group $G_k$ naturally embedded into $G_n$ are completely reducible with irreducible constituents $\phi_k^1$ and $\phi_k^2$, and their restrictions to $Sp_{2n}(p)$ can be obtained as the result of the reduction modulo $p$ of certain complex irreducible representations of the group $Sp_{2n}(p)$.