Vjylku

I'm doing something wrong in the code below that makes the last equation not be written on a line by itself. What I'm trying to do is to split the penultimate equation into two lines. But this happens:

The last equation continues on the right for some reason. Can you spot what is the problem?

usepackage{amsmath}

newcommand{q}[2]{langle #1 | #2 rangle}

newcommand{qr}[1]{|#1rangle} 

newcommand{ql}[1]{langle #1|} 

DeclareMathOperator{mdc}{mdc}



begin{document}

begin{align*}

  qr{psi_2} &= U_f qr{psi_1}\

              &= U_f frac{1}{sqrt{2^4}} left[qr0 + qr1 + qr2+qr3+qr4+qr5+qr6 + qr{7}right] qr0 qr0\

              &= U_f frac{1}{sqrt{2^4}} left[qr0qr0 + qr1qr0 + qr2qr0+qr3qr0+qr4qr0+qr5qr0+qr6qr0 + qr{7}qr0right] qr0\

begin{split}

              &= frac{1}{sqrt{2^4}} big[U_fqr0qr0 + U_fqr1qr0 + U_fqr2qr0+U_fqr3qr0 \

              &qquadqquad + U_fqr4qr0+U_fqr5qr0+U_fqr6qr0 + U_fqr{7}qr0big] qr0\

end{split}

              &= frac{1}{sqrt{2^4}} left[qr0qr2 + qr1qr4 + ... + qr{i}qr{f(i)} + ... + qr{7}qr1right] qr0,

end{align*}



end{document}

In addition to getting rid of the unneeded split environment, you may want to (a) typeset the U_f terms more compactly, i.e., as U_{!f}, (b) replace various left[ and right] directives with bigl[ and bigr], and (c) replace ... with dots.

documentclass{article}

usepackage{mathtools} % for 'DeclarePairedDelimiter' macro

DeclarePairedDelimiterqrlvertrangle

newcommand{Uf}{U_{!f}}



begin{document}



begin{align*}

qr{psi_2} 

&= Uf qr{psi_1}\

&= Uf frac{1}{sqrt{2^4}} 

   bigl[qr0 + qr1 + qr2+qr3 +qr4 

   +qr5 +qr6 + qr7bigr] qr0 qr0\

&= Uf frac{1}{sqrt{2^4}} 

   bigl[qr0qr0 + qr1qr0 + qr2qr0+qr3qr0

   +qr4qr0+qr5qr0+qr6qr0 + qr7qr0bigr] qr0\

&= frac{1}{sqrt{2^4}} 

   bigl[Ufqr0qr0 + Ufqr1qr0 + Ufqr2qr0+Ufqr3qr0 \

&qquadqquad 

   + Ufqr4qr0+Ufqr5qr0+Ufqr6qr0 

   + Ufqr7qr0bigr] qr0\

&= frac{1}{sqrt{2^4}} 

   bigl[qr0qr2 + qr1qr4 + dots 

   + qr{i}qr{f(i)} + dots + qr7qr1bigr] qr0,

end{align*}

end{document}

I suggest using the multlined environment from mathtools, and the DeclarePairedDelimiter command from the same package, which will produce scaling , qr and ql:

documentclass{article}

usepackage{mathtools}

DeclarePairedDelimiterXnorm[1]lVertrVert{ifblank{#1}{: · :}{#1}}

DeclarePairedDelimiterXPP{twonorm }[1]{}{lVert}{rVert}{_{2}}{ifblank{#1}{: · :}{#1}}

DeclarePairedDelimiterXq[2]{langle}{rangle}%

{#1,delimsizevert,mathopen{}#2}

DeclarePairedDelimiter{qr}vertrangle

DeclarePairedDelimiter{ql}langle rvert

DeclareMathOperator{mdc}{mdc}



begin{document}



begin{align*}

  qr{psi_2} &= U_f qr{psi_1}\

              &= U_f frac{1}{sqrt{2^4}} bigl[qr0 + qr1 + qr2+qr3+qr4+qr5+qr6 + qr{7}bigr] qr0 qr0\

              &= U_f frac{1}{sqrt{2^4}}bigl[qr0qr0 + qr1qr0 + qr2qr0+qr3qr0+qr4qr0+qr5qr0+qr6qr0 + qr{7}qr0bigr] qr0\

              &= begin{multlined}[t] frac{1}{sqrt{2^4}} bigl[U_fqr0qr0 + U_fqr1qr0 + U_fqr2qr0+U_fqr3qr0 \[-1.5ex]

             qquad+ U_fqr4qr0+U_fqr5qr0+U_fqr6qr0 + U_fqr{7}qr0bigr] qr0

end{multlined}\

              &= frac{1}{sqrt{2^4}}bigl[qr0qr2 + qr1qr4 + ... + qr{i}qr{f(i)} + ... + qr{7}qr1bigr] qr0,

end{align*}



end{document} 

I think the reason you are getting this result is that the second linebreak inside the split environment should be outside, like this:

begin{align*}

  qr{psi_2} &= U_f qr{psi_1}\

              &= U_f frac{1}{sqrt{2^4}} left[qr0 + qr1 + qr2+qr3+qr4+qr5+qr6 + qr{7}right] qr0 qr0\

              &= U_f frac{1}{sqrt{2^4}} left[qr0qr0 + qr1qr0 + qr2qr0+qr3qr0+qr4qr0+qr5qr0+qr6qr0 + qr{7}qr0right] qr0\

begin{split}

              &= frac{1}{sqrt{2^4}} big[U_fqr0qr0 + U_fqr1qr0 + U_fqr2qr0+U_fqr3qr0 \

              &qquadqquad + U_fqr4qr0+U_fqr5qr0+U_fqr6qr0 + U_fqr{7}qr0big] qr0

end{split}\

              &= frac{1}{sqrt{2^4}} left[qr0qr2 + qr1qr4 + ... + qr{i}qr{f(i)} + ... + qr{7}qr1right] qr0,

end{align*}

Besides the recomendations given in Bernard's answer, I'd also suggest using the split environment inside equation* instead of align*, since it seems you're trying to typeset just one (albeit multi-line) equation:

documentclass{article}

usepackage{amsmath}

newcommand{q}[2]{langle #1 | #2 rangle}

newcommand{qr}[1]{|#1rangle} 

newcommand{ql}[1]{langle #1|} 

DeclareMathOperator{mdc}{mdc}



begin{document}



begin{equation*}

 begin{split}

  qr{psi_2} &= U_f qr{psi_1}\

              &= U_f frac{1}{sqrt{2^4}} left[qr0 + qr1 + qr2+qr3+qr4+qr5+qr6 + qr{7}right] qr0 qr0\

              &= U_f frac{1}{sqrt{2^4}} left[qr0qr0 + qr1qr0 + qr2qr0+qr3qr0+qr4qr0+qr5qr0+qr6qr0 + qr{7}qr0right] qr0\

              &=!begin{multlined}[t] frac{1}{sqrt{2^4}} big[U_fqr0qr0 + U_fqr1qr0 + U_fqr2qr0+U_fqr3qr0 \[-1.5ex]

             qquad+ U_fqr4qr0+U_fqr5qr0+U_fqr6qr0 + U_fqr{7}qr0big] qr0

end{multlined}\

              &= frac{1}{sqrt{2^4}} left[qr0qr2 + qr1qr4 + ... + qr{i}qr{f(i)} + ... + qr{7}qr1right] qr0,

 end{split}

end{equation*}



end{document}