# Package amsmath Error: \begin{multline} allowed only in paragraph mode

I am getting an error message when I try to use the multline environment inside cases. This is my equation:

$$\label{eq:SMSKIN_TU2009} \begin{split} SM_{SKIN,Int} &= SM_{SKIN,Old} + INI - Q_M - EA_{SKIN} + SM_{SKIN,Upd} \\ SM_{SKIN,New} &= \begin{cases} FC_{SKIN} & \begin{multline} \quad \text{if } SM_{SKIN,Int} > FC_{SKIN}, \quad Q_{SKIN} = SM_{SKIN,Int} - \\ FC_{SKIN} \end{multline} \\ 0 & \quad \text{if } SM_{SKIN,Int} < 0, \quad Q_{SKIN} = 0\\ SM_{SKIN,Int} & \quad \text{otherwise }, \quad Q_{SKIN} = 0\\ \end{cases} \end{split}$$


If I don't use the multline environment, the equation is correctly displayed but the compiler throws a warning since the line doesn't fit the page.

I appreciate all your help. Thanks!

• It should not use multline in any case, try aligned probably best with the [t] option – daleif Aug 2 at 14:05
• aligned also shows errors... can you please provide a more explicit answer @daleif ? – Rodolfo Aug 2 at 14:21
• Aligned works just fine, would you please provide a full minimal example including preamble and document class, I just added document class and amsmath and replaced multline with aligned, compiles out of the box. – daleif Aug 2 at 14:25
• You might also want to have a look at the mathtools package and at its multlined environment. – GuM Aug 2 at 15:39

As @GuM has suggested, you could load the mathtools package (a superset of the amsmath package) and use its multlined environment instead of the illegal multline environment However, I think you're better off using a simple \parbox to display the material in question.
I would also use \mathit to typeset the multi-letter variable names and \mathrm to typeset the acronyms placed in the subscripts.
\documentclass{article}
$$\label{eq:SMSKIN_TU2009} \begin{split} \mathit{SM}_{\mathrm{SKIN},\mathrm{Int}} &= \mathit{SM}_{\mathrm{SKIN},\mathrm{Old}} + \mathit{INI} - Q_M - \mathit{EA}_{\mathrm{SKIN}} + \mathit{SM}_{\mathrm{SKIN},\mathrm{Upd}} \\ \mathit{SM}_{\mathrm{SKIN},\mathrm{New}} &= \begin{dcases} \mathit{FC}_{\mathrm{SKIN}} %% set up a parbox with automatic hanging indentation from the 2nd line onward &\parbox[t]{6cm}{\raggedright\hangafter1\hangindent1em if \mathit{SM}_{\mathrm{SKIN},\mathrm{Int}} > \mathit{FC}_{\mathrm{SKIN}}, Q_{\mathrm{SKIN}} = \mathit{SM}_{\mathrm{SKIN},\mathrm{Int}} - \mathit{FC}_{\mathrm{SKIN}}} \\ 0 & \text{if \mathit{SM}_{\mathrm{SKIN},\mathrm{Int}} < 0, Q_{\mathrm{SKIN}} = 0}\\ \mathit{SM}_{\mathrm{SKIN},\mathrm{Int}} & \text{otherwise, Q_{\mathrm{SKIN}} = 0} \end{dcases} \end{split}$$