# Separate a long equation in an array environment

I wanna to separate a long equation in an array environment, as it shows in the following graph: Place the right part of || to the below of =.

CODE:

$\Delta P_k^{ij} \; \text{is} \; \left\{ \begin{array}{l l} \text{known} & \text{if \Delta P_k=\bf{0}}\\ \bf{0} & \text{if \Delta P_k=-(I-K_k^i H_k^i)( F_{k-1} P^{ij}_{k-1} F^T_{k-1}+Q_{k-1})(I-K_k^j H_k^j)^T }\\ \text{unknown} & \text{otherwise}. \end{array} \right.$

• use the aligned environment from amsmath with the [t] option. Aug 21, 2015 at 14:23

Just an extension to Barbaras comment

Note I've replaced \bf as (1) it should not be used, (2) does not take arguments

\documentclass[a4paper]{memoir}
\usepackage{amsmath,amssymb}
\begin{document}

\Delta P_k^{ij} \; \text{is} \begin{cases} \text{known} & \text{if \Delta P_k=\mathbf{0}}\\ \mathbf{0} & \text{if  \begin{aligned}[t] \Delta P_k={}&-(I-K_k^i H_k^i)( F_{k-1} P^{ij}_{k-1} F^T_{k-1} \\ &+Q_{k-1})(I-K_k^j H_k^j)^T \end{aligned}  }\\ \text{unknown} & \text{otherwise}. \end{cases}
\end{document}


• Adding {} after F_{k-1} and Q_{k-1} would lower the subscript terms, making them line up with the other subscript terms in the formula.
– Mico
Aug 21, 2015 at 14:33
• Perhaps writing `(K^i_k H^i_k - I)' reads nicer than '-(I - K^i_k H^i_k)'. It also simplifies the expression slightly. I'd also add one or two quads before the plus symbol or introduce a nicer break, e.g. at the multiplication level.
– user10274
Aug 21, 2015 at 14:34
• Feel free to update the answer Aug 21, 2015 at 14:47