Gfilui

I'm trying to align a system of symbolic equations with alignat command like this:

beginalignat5
 a_11x_1&+a_12x_2&+ dots&+ a_1nx_n&=b_1\
 a_21x_1&+a_22x_2&+ dots&+ a_2nx_n&=b_2\
 a_m1x_1&+a_m2x_2&+ dots&+ a_mnx_n&=b_m
endalignat

but the problem lies within the letter "m" in the b_m symbol. The result is as follows:

If, however I put a number instead of "m", for ex. b_3 the result looks fine:

beginalignat5
 a_11x_1&+a_12x_2&+ dots&+ a_1nx_n&=b_1\
 a_21x_1&+a_22x_2&+ dots&+ a_2nx_n&=b_2\
 a_m1x_1&+a_m2x_2&+ dots&+ a_mnx_n&=b_3
endalignat

I would appreciate if someone can elaborate on why this happens and how to get rid of this freak occurance.

alignat uses pairs columns with rl-alignment. In your example the last column is of type r. So the last column will be right aligned - see the commented line in the following code snippet:

beginalignat3% 3 r-columns 
% r & l & r & l & r 
 a_11x_1 & +a_12x_2 & + dots & + a_1nx_n & =b_1\
 a_21x_1 & +a_22x_2 & + dots & + a_2nx_n & =b_2\
 a_m1x_1 & +a_m2x_2 & + dots & + a_mnx_n & =b_m
endalignat

Note that the argument of the alginat environment takes the numbers of r columns.

To get the desired result you have to ensure that the columns get the right alignment. In the following example there ars two suggestions with the same result:

documentclassarticle
usepackageamsmath
begindocument
beginalignat3% 3 r-columns 
% r & l & r & l & r & l
 a_11x_1 & +a_12x_2 & + dots & + a_1nx_n & & =b_1\
 a_21x_1 & +a_22x_2 & + dots & + a_2nx_n & & =b_2\
 a_m1x_1 & +a_m2x_2 & + dots & + a_mnx_n & & =b_m
endalignat

beginalignat3% 3 r-columns 
% r & l & r & l & r & l
 a_11x_1 & +a_12x_2 & + dots & + a_1nx_n & = & b_1\
 a_21x_1 & +a_22x_2 & + dots & + a_2nx_n & = & b_2\
 a_m1x_1 & +a_m2x_2 & + dots & + a_mnx_n & = & b_m
endalignat
enddocument

Based on my comment:

documentclassarticle
usepackageamsmath
begindocument
beginalignat6
 a_11x_1&+a_12x_2&+ dots&+ a_1nx_n&=&b_1\
 a_21x_1&+a_22x_2&+ dots&+ a_2nx_n&=&b_2\
 a_m1x_1&+a_m2x_2&+ dots&+ a_mnx_n&=&b_m
endalignat
enddocument

With array:

documentclassarticle
usepackagearray,mathtools

begindocument

[
renewcommandarraystretch1.5
setlengtharraycolsep0pt
defbvdotsvdotswithin=
beginarray
 l l >c< % a_11 & x_1 & +
 l l >c< % a_12 & x_2 & +
 c >c< % cdots +
 l l >c< l % a_12 & x_2 & = & b_1

 a_11 & x_1 & + & a_12 & x_2 & + & cdots & + & a_1n & x_n & = & b_1 \
 a_21 & x_1 & + & a_22 & x_2 & + & cdots & + & a_2n & x_n & = & b_2 \
 & & & & & & & & & & bvdots \
 a_m1 & x_1 & + & a_m2 & x_2 & + & cdots & + & a_mn & x_n & = & b_m
endarray
]

enddocument

Just for fun, with autoaligne:

documentclassarticle
usepackageautoaligne

begindocument

[
definirseparateurs\ 
endlinechar=-1
defbvdotskern-2emvdots
autoaligne[*g]
 a_11 x_1+a_12 x_2+cdots+a_1n x_n=b_1\
 a_21 x_1+a_22 x_2+cdots+a_2n x_n=b_2\
 + + + V bvdots\
 a_m1 x_1+a_m2 x_2+cdots+a_mn x_n=b_m

]

enddocument