Alterar estrutura de itemizar/enumerar

Question

Eu tenho o código a seguir, que funciona e formata perfeitamente.

\documentclass[a4paper,11pt]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{classicthesis}
\usepackage[english]{babel}
\usepackage{mathtools}
\usepackage{enumitem}

\begin{document}

\section*{Examples}
For each function $f$ given,
\begin{enumerate}
\begin{enumerate}
\item Sketch the graph of $f.$
\item Write the open intervals for which $f$ is continuous.
\item Write the open intervals for which $f$ is increasing.
\item Write the open intervals for which $f$ is decreasing.
\item Write the open intervals for which $f$ is concave up.
\item Write the open intervals for which $f$ is concave down.
\end{enumerate}

\item $f(x)=x^3-x^2-x+1.$
\item $f(x)=\dfrac{x^2-2x+2}{x-1}.$
\item $f(x)=x\sqrt{3-x}.$
\item $f(x)=\dfrac{x^3}{36}\sqrt{7-x}.$
\end{enumerate}

\end{document}

Mas dá um erro de \item ausente. O resultado,

Como devo editar o código para evitar o erro, mas manter o mesmo formato e saída?

Answer 1

Você pode fazer o que quiser com um simples vazio \item:

\documentclass[a4paper,11pt]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{classicthesis}
\usepackage[english]{babel}
\usepackage{mathtools}
\usepackage{enumitem}

\begin{document}

\section*{Examples}
For each function $f$ given,
\begin{enumerate}
  \item[]%
        \begin{enumerate}[before = \vspace*{-\topsep}]
          \item Sketch the graph of $f.$
          \item Write the open intervals for which $f$ is continuous.
          \item Write the open intervals for which $f$ is increasing.
          \item Write the open intervals for which $f$ is decreasing.
          \item Write the open intervals for which $f$ is concave up.
          \item Write the open intervals for which $f$ is concave down.
        \end{enumerate}

  \item $f(x)=x^3-x^2-x+1.$
  \item $f(x)=\dfrac{x^2-2x+2}{x-1}.$
  \item $f(x)=x\sqrt{3-x}.$
  \item $f(x)=\dfrac{x^3}{36}\sqrt{7-x}.$
\end{enumerate}

\end{document} 

Answer 2

aqui está uma solução

\documentclass[a4paper,11pt]{book}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage{classicthesis}
\usepackage{hyperref}
\usepackage{graphicx}
\usepackage[english]{babel}
\usepackage{amsmath,mathtools}
\usepackage{amsfonts}
\usepackage{color}
\usepackage{url}
\usepackage{enumitem}
\usepackage{multicol}
\usepackage{tabu}
\usepackage{amsthm}
\usepackage{fourier-orns}
\usepackage{framed}
\usepackage{tcolorbox}
\usepackage[toc,page]{appendix}
\usepackage[all]{xy}

\begin{document}

\section*{Examples}
For each function $f$ given,
\begin{enumerate}[label=\alph*),leftmargin=2\leftmargini]
\item Sketch the graph of $f.$
\item Write the open intervals for which $f$ is continuous.
\item Write the open intervals for which $f$ is increasing.
\item Write the open intervals for which $f$ is decreasing.
\item Write the open intervals for which $f$ is concave up.
\item Write the open intervals for which $f$ is concave down.
\end{enumerate}

\begin{enumerate}
\item $f(x)=x^3-x^2-x+1.$
\item $f(x)=\dfrac{x^2-2x+2}{x-1}.$
\item $f(x)=x\sqrt{3-x}.$
\item $f(x)=\dfrac{x^3}{36}\sqrt{7-x}.$
\end{enumerate}

\end{document}