Tjyylli

I am trying to generate the following equation:

I am working with an old file (I have not written it) that already has the following code for this equation:

begin{equation}

delta(x)equivleft{allbegin{aligned}

0,quad

text{if};xne 0\

infty,quad

text{if}; x=0

end{aligned}right}quad

text{with}quadint_{-infty}^{+infty}delta(x);dx=1.

end{equation}   

But this code doesn't work. Do you know what is wrong with that?

As @siracusa has already pointed out in a comment, the all directive should be removed. The only other significant change I recommend making to your code is adding & alignment points inside the aligned environment. (After all, why employ an algin environment if no alignment is performed?)

documentclass{article}

usepackage{amsmath} % for 'aligned' environment

begin{document}



begin{equation}

delta(x)equiv

left{

begin{aligned}

0,     quad&text{if $xne 0$}\

infty,quad&text{if $x=0$}

end{aligned}

right},,

qquadtext{with }

int_{-infty}^{+infty}! delta(x),dx=1.

end{equation}   

end{document}

You can use cases to simplify it a little bit and, of course, remove all as @siracusa said.

documentclass{article}

usepackage{amsmath}

begin{document}



begin{equation}

delta(x)equivleft.begin{cases}

0,      & text{if } x ne 0 \

infty, & text{if } x = 0 

end{cases}right}, quadtext{with } int_{-infty}^{+infty}delta(x); dx = 1.

end{equation}



end{document}

The simplest way is probably the Bmatrix environment.

Unrelated: I took the opportunity to improve the spacing in the integral.

documentclass{article}

usepackage{mathtools}



begin{document}



begin{equation}

delta(x)equiv

begin{Bmatrix}

0, & text{if } x ne 0 \

infty, & text{if } x = 0

end{Bmatrix},

quadtext{with }int_{-infty}^{mathrlap{+infty}}delta(x),dx = 1.

end{equation}



end{document} 

You can compare four ways to get the result.

The alignment with the original code happens essentially by chance and needs visual formatting (additions of quad and ;).

With Bmatrix the alignment is again by chance (the second column items have the same width).

With cases, adding left. and right., a bigger space after equiv is used.

The best, in my opinion, is to use left{ and right}, with a nested aligned so you can clearly state the desired left alignment of both columns.

In any case, it should be text{if $x=0$}, so no awkward explicit ; is needed (which is not a normal interword space). Also , should precede dx, not ;.

documentclass{article}

usepackage{amsmath}



begin{document}



subsubsection*{Original without texttt{stringall}}

begin{equation}

delta(x)equivleft{begin{aligned}

0,quad

text{if};xne 0\

infty,quad

text{if}; x=0

end{aligned}right}quad

text{with}quadint_{-infty}^{+infty}delta(x);dx=1.

end{equation}  



subsubsection*{With texttt{Bmatrix}}

begin{equation}

delta(x)equiv

begin{Bmatrix}

0,      & text{if $xne 0$} \

infty, & text{if $x=0$}

end{Bmatrix}

quadtext{with}quad

int_{-infty}^{+infty}delta(x),dx=1.

end{equation}



subsubsection*{With texttt{cases}}

begin{equation}

delta(x)equiv

left.

begin{cases}

0,      & text{if $xne 0$} \

infty, & text{if $x=0$}

end{cases}right}

quadtext{with}quad

int_{-infty}^{+infty}delta(x),dx=1.

end{equation}



subsubsection*{With texttt{aligned}}

begin{equation}

delta(x)equiv

left{

begin{aligned}

&0,      && text{if $xne 0$} \

&infty, && text{if $x=0$}

end{aligned}

right}

quadtext{with}quad

int_{-infty}^{+infty}delta(x),dx=1.

end{equation}



end{document}