Ytukyg

Take a look at this here:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amsfonts}

usepackage{amssymb}

begin{document}

begin{align*}

Leftrightarrow qquad P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} && (2.6)\

Leftrightarrow qquad e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)&=lambda e^{lambda t} e^{-lambda t}=lambda \

end{align*}

end{document}

Is it possible to have the equivalences below each other like the equations are?

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amsfonts}

usepackage{amssymb}

begin{document}

begin{alignat*}{4}

P'_1(t)&=-lambda P_1(t)+lambda P_0(t)\

Leftrightarrow &&qquad  P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} && (2.6)\

Leftrightarrow &&  e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)&=lambda e^{lambda t} e^{-lambda t}=lambda \

end{alignat*}

end{document}

This almost works but i have the mention problem that the first line is pulled to the very left.

Like so?

documentclass{article}



usepackage{amsmath}

begin{document}

begin{align}

Leftrightarrow&& P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} tag{2.6} \

Leftrightarrow&& e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)&=lambda e^{lambda t} e^{-lambda t}=lambda notag

end{align}

end{document}

You also might be interested in the ArrowBetweenLines from mathtools:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{mathtools}

usepackage{amssymb}



numberwithin{equation}{section}



begin{document}



setcounter{section}{2}setcounter{equation}{5}



begin{equation}label{equivalenteqs}

begin{alignedat}{2}

 & & abc & =def \

 ArrowBetweenLines &qquad& P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} \

  ArrowBetweenLines &qquad& e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)

                           &=lambda e^{lambda t} e^{-lambda t}=lambda \

end{alignedat}

end{equation}

As we see in equation~eqref{equivalenteqs} we can blah blah.



end{document} 

You don't want to manually number your equations and not because of the fact that the numbers would be unaligned to each other, but because maintaining manual numbering is essentially impossible.

Use the label-ref mechanism for this.

Your problem can be solved with alignedat:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amssymb}



numberwithin{equation}{section}



begin{document}



% let's emulate being in section 2 with five

% numbered equations before this one; a real document

% will have nothing like this

setcounter{section}{2}setcounter{equation}{5} 



begin{equation}label{equivalenteqs}

begin{alignedat}{2}

  Leftrightarrow &qquad& P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} \

  Leftrightarrow &qquad& e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)

                           &=lambda e^{lambda t} e^{-lambda t}=lambda \

end{alignedat}

end{equation}

As we see in equation~eqref{equivalenteqs} we can blah blah.



end{document}

If you happen to have a first line without the arrows, just add the suitable number of alignment points:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amssymb}



numberwithin{equation}{section}



begin{document}



% let's emulate being in section 2 with five

% numbered equations before this one

setcounter{section}{2}setcounter{equation}{5} 



begin{equation}

begin{alignedat}{2}

                &qquad&  P'_1(t)&=-lambda P_1(t)+lambda P_0(t)\

Leftrightarrow &      &  P'_1(t)+lambda P_1(t)&=lambda e^{-lambda t} \

Leftrightarrow &      &  e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)&=lambda e^{lambda t} e^{-lambda t}=lambda \

Leftrightarrow &      & frac{d}{dt}(e^{lambda t}P_1(t))&=e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)=lambda\

end{alignedat}

end{equation}

As we see in equation~eqref{equivalenteqs} we can blah blah.



end{document}

On the other hand, I see no reason for aligning at the equals signs.

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amssymb}



numberwithin{equation}{section}



begin{document}



% let's emulate being in section 2 with five

% numbered equations before this one

setcounter{section}{2}setcounter{equation}{5} 



begin{equation}label{equivalenteqs}

begin{aligned}

                && & P'_1(t)=-lambda P_1(t)+lambda P_0(t)\

Leftrightarrow && & P'_1(t)+lambda P_1(t)=lambda e^{-lambda t} \

Leftrightarrow && & e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)=lambda e^{lambda t} e^{-lambda t}=lambda \

Leftrightarrow && & frac{d}{dt}(e^{lambda t}P_1(t))=e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t)=lambda\

end{aligned}

end{equation}

As we see in equation~eqref{equivalenteqs} we can blah blah.



end{document}

You can also use alignat:

Code:

documentclass[10pt,a4paper]{article}

usepackage[utf8]{inputenc}

usepackage{amsmath}

usepackage{amsfonts}

usepackage{amssymb}

begin{document}

begin{alignat*}{4}

  Leftrightarrow &&qquad P'_1(t)+lambda P_1(t) &=lambda e^{-lambda t} && (2.6) \

  Leftrightarrow &&qquad e^{lambda t}P'_1(t)+lambda e^{lambda t}P_1(t) &=lambda e^{lambda t} e^{-lambda t}=lambda 

end{alignat*}

end{document}