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Commit 476bf8a4 authored by Luke Naylor's avatar Luke Naylor
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Comment on bounds collapsing at extreme values of q

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......@@ -567,18 +567,21 @@ vertical wall (TODO as discussed in ref).
&+ \sage{bgmlv1_d_lowerbound_const_term_alt.subs(chbv == 0)}
+& \sage{bgmlv1_d_lowerbound_exp_term_alt.subs(chbv == 0)},
&\qquad\text{when\:} r > \frac{R}{2}
\label{eqn:bgmlv1_d_bound_betamin}
\\
d &\leq&
\sage{bgmlv2_d_upperbound_linear_term}
&+ \sage{bgmlv2_d_upperbound_const_term}
+& \sage{bgmlv2_d_upperbound_exp_term},
&\qquad\text{when\:} r > 0
\label{eqn:bgmlv2_d_bound_betamin}
\\
d &\leq&
\sage{bgmlv3_d_upperbound_linear_term}
&+ \sage{bgmlv3_d_upperbound_const_term_alt.subs(chbv == 0)}
+& \sage{bgmlv3_d_upperbound_exp_term_alt.subs(chbv == 0)},
&\qquad\text{when\:} r > R
\label{eqn:bgmlv3_d_bound_betamin}
\end{align}
Furthermore, we get an extra bound for $d$ resulting from the condition that the
......@@ -596,9 +599,10 @@ positive_radius_condition = (
)
\end{sagesilent}
\begin{equation*}
\begin{equation}
\label{eqn:positive_rad_d_bound_betamin}
\sage{positive_radius_condition}
\end{equation*}
\end{equation}
\begin{sagesilent}
def beta_min(chern):
ts = stability.Tilt()
......@@ -704,13 +708,13 @@ def plot_d_bound(
\begin{subfigure}{.5\textwidth}
\centering
\sageplot[width=\linewidth]{plot_d_bound(v_example, 0)}
\caption{$q = 0$}
\caption{$q = 0$ (all bounds other than green coincide on line)}
\label{fig:d_bounds_xmpl_min_q}
\end{subfigure}%
\begin{subfigure}{.5\textwidth}
\centering
\sageplot[width=\linewidth]{plot_d_bound(v_example, 4)}
\caption{$q = \chern^{\beta}(F)$}
\caption{$q = \chern^{\beta}(F)$ (all bounds other than blue coincide on line)}
\label{fig:d_bounds_xmpl_max_q}
\end{subfigure}
\caption{
......@@ -721,6 +725,21 @@ def plot_d_bound(
\label{fig:d_bounds_xmpl_extrm_q}
\end{figure}
Recalling that $q := \chern^{\beta}_1(E) \in [0, \chern^{\beta}_1(F)]$,
it's worth noting that the extreme values of $q$ in this range lead to the
tightest bounds on $d$, as illustrated in figure
(\ref{fig:d_bounds_xmpl_extrm_q}).
In fact, in each case, one of the weak upper bounds coincides with one of the
weak lower bounds, (implying no possible destabilizers $E$ with
$\chern_0(E)=:r>R:=\chern_0(F)$ for these $q$-values).
This indeed happens in general since the right hand sides of
(eqn \ref{eqn:bgmlv2_d_bound_betamin}) and
(eqn \ref{eqn:positive_rad_d_bound_betamin}) match when $q=0$.
In the other case, $q=\chern^{\beta}_1(F)$, it's the right hand sides of
(eqn \ref{eqn:bgmlv3_d_bound_betamin}) and
(eqn \ref{eqn:positive_rad_d_bound_betamin}) which match.
\begin{figure}
\centering
\sageplot[
......
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