Shannon entropy for harmonic metrics on cyclic Higgs bundles

Abstract: Let $X$ be a Riemann surface and $K_X \rightarrow X$ the canonical bundle. For each integer $r \geq 2$, each $q \in H^0(K_X^r)$, and each choice of the square root $K_X^{1/2}$ of the canonical bundle, we canonically obtain a Higgs bundle, which is called a cyclic Higgs bundle. A diagonal harmonic metric $h = (h_1, \dots, h_r)$ on a cyclic Higgs bundle yields $r-1$-Hermitian metrics $H_1, \dots, H_{r-1}$ on $K_X^{-1} \rightarrow X$, defined as $H_j=h_j^{-1} \otimes h_{j+1}$ for each $j=1,\dots, r-1$, while $h_1$, $h_r$, and $q$ yield a degenerate Hermitian metric $H_r$ on $K_X^{-1} \rightarrow X$. In this paper, we introduce a function, which we call entropy, that quantifies the degree of mutual misalignment of the Hermitian metrics $H_1,\dots, H_r$. By applying the estimate established by Dai-Li and Li-Mochizuki, we provide a strict upper bound for the entropy, a lower bound in the cases $r=2,3$, and a lower bound at the zero set of $q$ in the cases $r=4,5$. The $r$-differential $q$ induces a subharmonic weight $\phi_q$ on $K_X\rightarrow X$, and the diagonal harmonic metric depends solely on this weight $\phi_q$. We further generalize the concept of entropy to any subharmonic weight $\varphi$ on $K_X\rightarrow X$. By extending the estimate established by Dai-Li and Li-Mochizuki to any subharmonic weight under a certain regularity assumption, we generalize the estimate for the entropy for usual cyclic Higgs bundles to more general weights. Additionally, we compute the limit of the entropy as $r\to\infty$ for the Fuchsian case.