I'm trying to make a block of equations where the first two are standalone equations, and the second two are subequations, ie
X=XXXX (1)
X=XXXX (2)
X=XXXX (3a)
=XXXX (3b)
I'm using the amsmath package, with align and subequation, but I can't figure out how to do this without making a separate block for Eqs 1-2 and one for Eqs 3a-3b. That solution is unsatisfactory, because then there is a big vertical gap between the two blocks, and I want them to appear all as one.
Here's the code I have so far, in case it's unclear.
\begin{align}
T(x) &= \frac{m}{2{k_{B}}} \left\langle \mathbf{\tilde{v}}{(\mathbf{x}_{i},t)}^{2} \right\rangle\\
\mathbf{u}(x) &= \left\langle \mathbf{v}_{i}(\mathbf{x}_{i},t) \right\rangle
\end{align}
\begin{subequations}
\begin{align}
P_{xy}(x) &= \frac{1}{A} \left\langle \sum\limits_{i} {m \tilde{v}_{i,x} \tilde{v}_{i,y}} + \frac{1}{2} \sum\limits_{i} {\sum\limits_{j\ne i} {r_{ij,y} F_{ij,x}}} \right\rangle \\
\begin{split}
&=\frac{m}{A} \Bigg\langle \sum\limits_{i}{{{{\tilde{v}}}_{i,x}}{{{\tilde{v}}}_{i,y}}} + \frac{a^3\omega _{pd}^2}{4} \\
&\quad \times \sum\limits_{i}{\sum\limits_{j\ne i}{r_{ij,y}{\left\{ \frac{e^{(-{{r}_{ij}}/\lambda )}}{{{r}_{ij}}}\left( \frac{1}{r_{ij}}+\frac{1}{\lambda} \right) \right\}_{x}}}} \Bigg\rangle
\end{split}
\end{align}
\end{subequations}
Any suggestions would be greatly appreciated!