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I am using alignat environment to align a set of equations which practically describe the same thing, but for different quantities. Therefore I decided to skip the common things and replace them with cdots, however I don't seem to be able to align them in the center.

Any idea to center the dots?

A sample code follows

documentclass{article}

usepackage{amsmath}



begin{document}



begin{alignat*}{4}

  &sigma(E) &=& vec{sigma(E)} = big{sigma(E_1), sigma(E_2), cdots, sigma(E_n)big} &=&  big{sigma_1, sigma_2, cdots, sigma_nbig}&\

 &C(E) &=&{cdots}&=&  big{C_1, C_2, cdots, C_nbig}&\

end{alignat*}



end{document}

You can result to low level TeX with the omit to break out of the current formatting:

documentclass{article}



usepackage{amsmath}



begin{document}



begin{alignat*}{2}

  sigma(E) &={}& vec{sigma(E)}

  = bigl{sigma(E_1), sigma(E_2), dots, sigma(E_n)bigr}

  &= bigl{sigma_1, sigma_2, dots, sigma_nbigr}\

  C(E) &={}&omithfill$cdots$hfill

  &= bigl{C_1, C_2, dots, C_nbigr}

end{alignat*}



end{document}

Note I have also added groups {} to get correct spacing at the equals signs, and use bigl...bigr instead of big. Also between commas you should just use dots (or dotsc).

Be careful here with the use of the ampersands &. In alignat, the alignment is implicitly rlrl... with the first group right aligned, the second left aligned, etc.

A solution with the eqparbox package. Note that 2 alignment columns are enough.

documentclass{article}

usepackage{amsmath}

usepackage{eqparbox}

newcommand{eqmathbox}[2][M]{eqmakebox[#1]{$displaystyle#2$}}



begin{document}



begin{alignat*}{2}

  sigma(E) &= {}& eqmathbox{vec{sigma(E)} = big{sigma(E_1), sigma(E_2), cdots, sigma(E_n)big}} &= big{sigma_1, sigma_2, cdots, sigma_nbig}\

 C(E) &= {}& eqmathbox{cdotscdots}&= big{C_1, C_2, cdots, C_nbig}\

end{alignat*}



end{document} 

array is not made for aligning equations, but in this case, with a couple of tricks (@{} and {} where we want a correct spacing after/before the first/last equal sign), it may be convenient.

documentclass{article}

usepackage{amsmath}

usepackage{array}

begin{document}

[

begin{array}{r@{}c@{}l}

  sigma(E) ={} & vec{sigma(E)} = big{sigma(E_1), sigma(E_2), cdots, sigma(E_n)big} & {}= big{sigma_1, sigma_2, cdots, sigma_nbig}\

 C(E) ={} &cdots& {}=  big{C_1, C_2, cdots, C_nbig}\

end{array}

]

end{document}