Section headers disappear in TeXLive 2026

My code snippets:

\renewcommand{\chaptermark}[1]{\markboth{\scshape\chaptername\ \thechapter. #1}{}}%
\renewcommand{\sectionmark}[1]{\markright{\scshape\S\thesection. #1}}%

worked just fine in TeXLive 2022 (using amsbook format in LaTeX), but in TeXLive 2026 the section heads do not appear at all. I gather this is due to the changes in the way headers are handled now.

Can you suggest a modern modification I can make to get section heads to appear? fancyhdr did nothing for me.

EDIT: OK. Here is a short document which typesets to three pages. Using TeXLive 2022 I get a section header on p. 3; using TeXLive 2026 there is none. :)

\documentclass[11pt]{amsbook}

\renewcommand{\chaptermark}[1]{\markboth{\scshape\chaptername\ \thechapter. #1}{}}%
\renewcommand{\sectionmark}[1]{\markright{\scshape\S\thesection. #1}}%


\begin{document}
\chapter{Curves}

\section{Examples, Arclength Parametrization}
We say a vector function $f\: (a,b)\to\mathbb R^3$ is $C^k$
($k=0,1,2,\dots$) if $f$ and its first $k$ derivatives, $f'$, $f''$, \dots,
$f^{(k)}$, exist and are all continuous. We say $f$ is smooth if $f$ is
$C^k$ for every positive integer $k$. A parametrized curve is a
$C^3$ (or smooth) map $\alpha\: I\to\mathbb R^3$ for some interval $I=(a,b)$ or $[a,b]$
(possibly infinite). We say $\alpha$ is regular if
$\alpha'(t)\ne 0$ for all $t\in I$.

We can imagine a particle moving along the path $\alpha$, with its position at time
$t$ given by $\alpha(t)$. As we learned in vector calculus,
$$\alpha'(t) =  \lim_{h\to 0}\frac{\alpha(t+h)-\alpha(t)}h$$
is the velocity of the particle at time $t$. The velocity vector
$\alpha'(t)$ is tangent to the curve at $\alpha(t)$ and its length, $\|\alpha'(t)\|$, is
the speed of the particle.

We say a vector function $f\: (a,b)\to\mathbb R^3$ is $C^k$
($k=0,1,2,\dots$) if $f$ and its first $k$ derivatives, $f'$, $f''$, \dots,
$f^{(k)}$, exist and are all continuous. We say $f$ is smooth if $f$ is
$C^k$ for every positive integer $k$. A parametrized curve is a
$C^3$ (or smooth) map $\alpha\: I\to\mathbb R^3$ for some interval $I=(a,b)$ or $[a,b]$
(possibly infinite). We say $\alpha$ is regular if
$\alpha'(t)\ne 0$ for all $t\in I$.

We can imagine a particle moving along the path $\alpha$, with its position at time
$t$ given by $\alpha(t)$. As we learned in vector calculus,
$$\alpha'(t) =  \lim_{h\to 0}\frac{\alpha(t+h)-\alpha(t)}h$$
is the velocity of the particle at time $t$. The velocity vector
$\alpha'(t)$ is tangent to the curve at $\alpha(t)$ and its length, $\|\alpha'(t)\|$, is
the speed of the particle.

We say a vector function $f\: (a,b)\to\mathbb R^3$ is $C^k$
($k=0,1,2,\dots$) if $f$ and its first $k$ derivatives, $f'$, $f''$, \dots,
$f^{(k)}$, exist and are all continuous. We say $f$ is smooth if $f$ is
$C^k$ for every positive integer $k$. A parametrized curve is a
$C^3$ (or smooth) map $\alpha\: I\to\mathbb R^3$ for some interval $I=(a,b)$ or $[a,b]$
(possibly infinite). We say $\alpha$ is regular if
$\alpha'(t)\ne 0$ for all $t\in I$.

We can imagine a particle moving along the path $\alpha$, with its position at time
$t$ given by $\alpha(t)$. As we learned in vector calculus,
$$\alpha'(t) =  \lim_{h\to 0}\frac{\alpha(t+h)-\alpha(t)}h$$
is the velocity of the particle at time $t$. The velocity vector
$\alpha'(t)$ is tangent to the curve at $\alpha(t)$ and its length, $\|\alpha'(t)\|$, is
the speed of the particle.

We say a vector function $f\: (a,b)\to\mathbb R^3$ is $C^k$
($k=0,1,2,\dots$) if $f$ and its first $k$ derivatives, $f'$, $f''$, \dots,
$f^{(k)}$, exist and are all continuous. We say $f$ is smooth if $f$ is
$C^k$ for every positive integer $k$. A parametrized curve is a
$C^3$ (or smooth) map $\alpha\: I\to\mathbb R^3$ for some interval $I=(a,b)$ or $[a,b]$
(possibly infinite). We say $\alpha$ is regular if
$\alpha'(t)\ne 0$ for all $t\in I$.

We can imagine a particle moving along the path $\alpha$, with its position at time
$t$ given by $\alpha(t)$. As we learned in vector calculus,
$$\alpha'(t) =  \lim_{h\to 0}\frac{\alpha(t+h)-\alpha(t)}h$$
is the velocity of the particle at time $t$. The velocity vector
$\alpha'(t)$ is tangent to the curve at $\alpha(t)$ and its length, $\|\alpha'(t)\|$, is
the speed of the particle.

We say a vector function $f\: (a,b)\to\mathbb R^3$ is $C^k$
($k=0,1,2,\dots$) if $f$ and its first $k$ derivatives, $f'$, $f''$, \dots,
$f^{(k)}$, exist and are all continuous. We say $f$ is smooth if $f$ is
$C^k$ for every positive integer $k$. A parametrized curve is a
$C^3$ (or smooth) map $\alpha\: I\to\mathbb R^3$ for some interval $I=(a,b)$ or $[a,b]$
(possibly infinite). We say $\alpha$ is regular if
$\alpha'(t)\ne 0$ for all $t\in I$.

We can imagine a particle moving along the path $\alpha$, with its position at time
$t$ given by $\alpha(t)$. As we learned in vector calculus,
$$\alpha'(t) =  \lim_{h\to 0}\frac{\alpha(t+h)-\alpha(t)}h$$
is the velocity of the particle at time $t$. The velocity vector
$\alpha'(t)$ is tangent to the curve at $\alpha(t)$ and its length, $\|\alpha'(t)\|$, is
the speed of the particle.



\section{Local Theory: Frenet Frame}
What distinguishes a circle or a helix from a line is their curvature,
i.e., the tendency of the curve to change direction. We shall now see that we
can associate to each smooth ($C^3$) arclength-parametrized curve $\alpha$ a
natural ``moving frame" (an orthonormal basis for $\mathbb R^3$ chosen at each point on
the curve, adapted to the geometry of the curve as much as possible).

We begin with a fact from vector calculus that will appear throughout this
course.

Suppose $f, g: (a,b)\to\mathbb R^3$
are differentiable and satisfy $f(t)\cdot g(t)=\text{const}$ for all $t$.
Then $f'(t)\cdot  g(t) = -f(t)\cdot g'(t)$. In particular,
$$\|f(t)\|=\text{const} \quad\text{if and only if}\quad f(t)\cdot f'(t)=0
\quad\text{for all }t\,.$$







What distinguishes a circle or a helix from a line is their curvature,
i.e., the tendency of the curve to change direction. We shall now see that we
can associate to each smooth ($C^3$) arclength-parametrized curve $\alpha$ a
natural ``moving frame" (an orthonormal basis for $\mathbb R^3$ chosen at each point on
the curve, adapted to the geometry of the curve as much as possible).

We begin with a fact from vector calculus that will appear throughout this
course.

Suppose $f, g: (a,b)\to\mathbb R^3$
are differentiable and satisfy $f(t)\cdot g(t)=\text{const}$ for all $t$.
Then $f'(t)\cdot  g(t) = -f(t)\cdot g'(t)$. In particular,
$$\|f(t)\|=\text{const} \quad\text{if and only if}\quad f(t)\cdot f'(t)=0
\quad\text{for all }t\,.$$


What distinguishes a circle or a helix from a line is their curvature,
i.e., the tendency of the curve to change direction. We shall now see that we
can associate to each smooth ($C^3$) arclength-parametrized curve $\alpha$ a
natural ``moving frame" (an orthonormal basis for $\mathbb R^3$ chosen at each point on
the curve, adapted to the geometry of the curve as much as possible).

We begin with a fact from vector calculus that will appear throughout this
course.

Suppose $f, g: (a,b)\to\mathbb R^3$
are differentiable and satisfy $f(t)\cdot g(t)=\text{const}$ for all $t$.
Then $f'(t)\cdot  g(t) = -f(t)\cdot g'(t)$. In particular,
$$\|f(t)\|=\text{const} \quad\text{if and only if}\quad f(t)\cdot f'(t)=0
\quad\text{for all }t\,.$$


What distinguishes a circle or a helix from a line is their curvature,
i.e., the tendency of the curve to change direction. We shall now see that we
can associate to each smooth ($C^3$) arclength-parametrized curve $\alpha$ a
natural ``moving frame" (an orthonormal basis for $\mathbb R^3$ chosen at each point on
the curve, adapted to the geometry of the curve as much as possible).

We begin with a fact from vector calculus that will appear throughout this
course.

Suppose $f, g: (a,b)\to\mathbb R^3$
are differentiable and satisfy $f(t)\cdot g(t)=\text{const}$ for all $t$.
Then $f'(t)\cdot  g(t) = -f(t)\cdot g'(t)$. In particular,
$$\|f(t)\|=\text{const} \quad\text{if and only if}\quad f(t)\cdot f'(t)=0
\quad\text{for all }t\,.$$


What distinguishes a circle or a helix from a line is their curvature,
i.e., the tendency of the curve to change direction. We shall now see that we
can associate to each smooth ($C^3$) arclength-parametrized curve $\alpha$ a
natural ``moving frame" (an orthonormal basis for $\mathbb R^3$ chosen at each point on
the curve, adapted to the geometry of the curve as much as possible).

We begin with a fact from vector calculus that will appear throughout this
course.

Suppose $f, g: (a,b)\to\mathbb R^3$
are differentiable and satisfy $f(t)\cdot g(t)=\text{const}$ for all $t$.
Then $f'(t)\cdot  g(t) = -f(t)\cdot g'(t)$. In particular,
$$\|f(t)\|=\text{const} \quad\text{if and only if}\quad f(t)\cdot f'(t)=0
\quad\text{for all }t\,.$$

\end{document}

I hope this helps.