Alignment and labeling list of equations

Question

I want to obtain a list of equations aligned on the left, where every equation has the labeling number on the right and it can be done by using align. However I have an equation which takes two lines and I want only one labeling number for the whole equation. How could I do? Thanks

EDIT:

\documentclass[a4paper,twoside,openright]{book}   
\usepackage[utf8x]{inputenc}   
\usepackage[T1]{fontenc}   
\usepackage[pdftex]{color,graphicx}  
\usepackage{amsthm, amssymb, amsmath,mathrsfs}  
\usepackage{caption}  
\usepackage{subfig}  
\usepackage{fancyhdr}  
\usepackage[italian]{babel}  
\usepackage{indentfirst}  
\usepackage{microtype}  
\usepackage{varioref}  
\usepackage{hyperref}  
\usepackage{comment}  
\usepackage{amsmath}  

\begin{document}
\begin{align}  
&(\lambda_0,\lambda(t))\neq(0,0) \label{eq:nontriv}\\  
&\text{Maximum principle: } u^{\ast}(t) \text { maximizes } \mathcal{H}(x^{\ast}(t),u(t),t), \text { for } u\in U \notag\\  
&\text {that is } \mathcal{H}(x^{\ast}(t),u^{\ast} 
 (t),\lambda(t),t)\ge\mathcal{H}(x^{\ast}(t),u,\lambda(t),t) \label{eq:pmax}\\  
(...etc...)  
\end{align}  

\end{document}

I wanted to enumerate only the first and the third line and now I got it. However I'd also like to gather the second and the third.

Answer 1

Some general options you might have to enumerate two equations as one:

\documentclass[]{article}

\usepackage{amsmath}

\begin{document}
Using subeqations to give them number 3a and 3b, also inserts more vertical
space.
\begin{align}
  E &= mc^2\\
  E &= \frac{1}{2}mv^2
\end{align}
\begin{subequations}
  \begin{align}
    \Delta E &= E_0 - E_1\\
    \Delta E &> 0
  \end{align}
\end{subequations}
Using split, so they share a number which is printed vertically between the two
equations.
\begin{align}
  E &= mc^2\\
  E &= \frac{1}{2}mv^2\\
  \begin{split}
    \Delta E &= E_0 - E_1\\
    \Delta E &> 0
  \end{split}
\end{align}
\end{document}

Answer 2

Since the second "equation" consists of a couple of sentences that contain inline math terms, it's a good idea to place the material in a \parbox. Observe that the associated equation number is centered vertically on the contents of the (two-line) parbox.

\documentclass[a4paper,twoside,openright]{book}   
\usepackage[utf8x]{inputenc}   
\usepackage[T1]{fontenc}   
\usepackage{amsthm, amssymb, amsmath,mathrsfs}  
%% The following packages aren't needed for this example:
%\usepackage{xcolor,graphicx}   % omit the 'pdftex' option
%\usepackage{caption}  
%\usepackage{subfig}  
%\usepackage{fancyhdr}  
%\usepackage[italian]{babel}  
%\usepackage{indentfirst}  
%\usepackage{microtype}  
%\usepackage{varioref}  
%\usepackage{hyperref}  
%\usepackage{comment}  
%\usepackage{amsmath}   % no need to load a package twice

\begin{document}
\hrule % just to illustrate width of textblock
\begin{align}  
&(\lambda_0,\lambda(t))\neq(0,0) \label{eq:nontriv}\\ 
&\parbox{0.85\textwidth}{\raggedright
Maximum principle: $u^{\ast}(t)$ maximizes 
$\mathcal{H}(x^{\ast}(t),u(t),t)$ for $u\in U$. 
That is, $\mathcal{H}(x^{\ast}(t),u^{\ast} (t),\lambda(t),t)\ge
\mathcal{H}(x^{\ast}(t),u,\lambda(t),t)$.} \label{eq:pmax}\\  
&\text{(\dots etc\dots) } \notag
\end{align}  

\end{document}