This introductory block (before the first chapter) is always visible.

Chapter 1 — Always open by default

Some math inline (E = mc^2) and display:

\[H = H_0 + V.\]

Section 1.1 — Collapsible

Subsection 1.1.1 — Visible once Section 1.1 is expanded

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Subsection 1.1.2

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Section 1.2 — Collapsible

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Chapter 2 — Collapsed by default

Section 2.1 — Collapsible

Subsection 2.1.1

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