# How to cite theorems within a series of equations?

I want to know if it's possible to cite theorems within series of equations, without interleaving text like "From Theorem 1.3, it follows that:". Here's an idea of the kind of formatting I'm looking for:

x = 1                                                (Theorem 1.1)
x^2 = 1                                              (Theorem 1.2)
x^2 - 1 = 0                                          (Theorem 1.3)
(x + 1)(x - 1) = 0                                   (Theorem 1.4)
x + 1 = 0                                            (Theorem 1.5)
1 + 1 = 0                                            (Theorem 1.6)
2 = 0                                                (Theorem 1.7)


I also want to be able to do this in a single long, chained equation. e.g. Intermixing it with the align* environment from amsmath:

expr1 = expr2                                        (Theorem 2.1)
= expr3                                        (Theorem 2.2)
= expr4                                        (Theorem 2.3)
= expr5                                        (Theorem 2.4)
= ...


Is it possible to achieve this?

• Yes it works. You can the the flalign environment for this, with the first column empty, the equations in the second column and the references in the last column. Nov 1, 2017 at 22:26
• @Bernard Sounds great. Can you provide an example via an answer? Nov 1, 2017 at 22:40

Here are 3 possible layouts for references. alignat gives you full control on the spacing between the equations and the reference, whereas flalign puts references at the right margin:

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{fourier}
\usepackage{mathtools}
\usepackage[standard, amsmath, thmmarks]{ntheorem}
\usepackage{cleveref}

\begin{document}
\begin{theorem}\label{th:P}
A powerful theorem.
\end{theorem}

\begin{theorem}\label{th:PP}
Another powerful theorem.
\end{theorem}

\begin{flalign}
& & a + b & = c + d + e& & \text{(by \cref{th:P})}\\
& & a' + b' + e'& = c'+ d' & & \text{(by \cref{th:PP})}
\end{flalign}

\begin{align}
a' + b' + e' & = c' + d' & & \text{(by \cref{th:PP})} \\
a + b & = c + d + e & & \text{(by \cref{th:P})}
\end{align}

\begin{alignat}{2}
a' + b' + e' & = c' + d' & \qquad & \text{(by \cref{th:PP})} \\
a + b & = c + d + e & & \text{(by \cref{th:P})}
\end{alignat}

\end{document}