# aligning equation

Regarding the enclosed MWE I would like to get the \sums aligned. But using a second & in lines two and three leads of course to a problem with line one. How do I get the desired result? (Looking into Herbert's fine mathmode was to no avail.)

\documentclass{scrartcl}
\usepackage{amsmath}
\begin{document}
\begin{align*}
S(f) & ={} \pi\left(S(f-f_0)+S(f+f_0)\right)\\
& ={} \pi\tau\left(\sum_{k=-\infty}^{+\infty} c_k \delta\left(\dfrac{k}{\tau}-f_0\right)+\right.\\
&                      \left.\sum_{k=-\infty}^{+\infty} c_k \delta\left(\dfrac{k}{\tau}+f_0\right)\right)
\end{align*}
\end{docum

• BTW: why ={}? Mar 13, 2015 at 7:55
• I experienced that otherwise there is (sometimes?) a missing distance between the = and the following term. Maybe that it is not necessary anymore ... Mar 13, 2015 at 8:25

\phantom{= \pi\tau\left(\vphantom{\sum_{k=-\infty}^{+\infty}}\right.}\kern-\nulldelimiterspace


after the & like in

\documentclass{scrartcl}
\usepackage{amsmath}

\begin{document}
\begin{align*}
S(f) & = \pi\left(S(f-f_0)+S(f+f_0)\right)\\
& = \pi\tau\left(\sum_{k=-\infty}^{+\infty} c_k \delta\left(\dfrac{k}{\tau}-f_0\right)+\right.\\
& \phantom{= \pi\tau\left(\vphantom{\sum_{k=-\infty}^{+\infty}}\right.}\kern-\nulldelimiterspace
\left.\sum_{k=-\infty}^{+\infty} c_k \delta\left(\dfrac{k}{\tau}+f_0\right)\right)
\end{align*}
\end{document}


With \biggl etc, it would be

\documentclass{scrartcl}
\usepackage{amsmath}

\begin{document}
\begin{align*}
S(f) & = \pi\left(S(f-f_0)+S(f+f_0)\right)\\
& = \pi\tau\Biggl(\sum_{k=-\infty}^{+\infty} c_k \delta\biggl(\dfrac{k}{\tau}-f_0\biggr)+\\
& \phantom{{}={}\pi\tau\Biggl(}\kern-\nulldelimiterspace
\sum_{k=-\infty}^{+\infty} c_k \delta\biggl(\dfrac{k}{\tau}+f_0\biggr)\Biggr)
\end{align*}
\end{document}


And with aligned as suggested by dalief

\documentclass{scrartcl}
\usepackage{amsmath}

\begin{document}
\begin{align*}
S(f) & = \pi\left(S(f-f_0)+S(f+f_0)\right)\\
& = \pi\tau\!
\begin{aligned}[t]
\Biggl(&\sum_{k=-\infty}^{+\infty} c_k \delta\biggl(\dfrac{k}{\tau}-f_0\biggr)+\\
& \sum_{k=-\infty}^{+\infty} c_k \delta\biggl(\dfrac{k}{\tau}+f_0\biggr)\Biggr)
\end{aligned}
\end{align*}
\end{document}


It is better to use the last option.

• Just as a comment to the OP: Or (since it is align*) use aligned to align the stuff inside the big ()'s. Also not always a good idea to use those broken \left...\right. ... \left. ... \right) constructions Mar 13, 2015 at 7:56
• @daleif It is a good idea. Thanks and added.
– user11232
Mar 13, 2015 at 8:09
• I don't think you need dual \!. I usually also include the starting ( in aligned when I do this. Then I know the spacing inside the ()'s will be ok and I only need the traditional \! in front. Mar 13, 2015 at 8:15
• @daleif That sounds good too. :)
– user11232
Mar 13, 2015 at 8:17
• My answer would have just the second option. ;-) I agree with daleif that \!\! is not needed and, in this particular case, neither the single \! is, because of the protruding “k”. Mar 13, 2015 at 8:18