A set $D$ is called a phase diagram if ...... [No comma before if here.]
The sequence $a_n$ is what is sometimes called a recovery sequence for $v$.
Call a set a phase diagram if ......
Call $H$ the width of $g$. [Note that this phrase defines the last term appearing (width), and $H$ is assumed to be known; cf. Denote by $H$ the width of $g$, which defines the notation $H$.]
The lemma motivates our calling $R$ a generalized Picard group.
This corollary calls for some explanation and comment.
We will call on this version of the inverse theorem when we come to our applications in Section 2.
Adding $E$ to both sides of (1), we can call upon (2) to obtain (3).