Etydhv

I would like to produce the following alignment with beginalign*...endalign*.

 -0,1 < E < 0,1 
 6,3 < S < 6,4 
6,3-0,1 < S+E < 6,4+0,1
 6,2 < I < 6,5

I can't figure out where to place the & signs.
Thanks for your help.

documentclassarticle
usepackageamsmath

begindocument



alignat:
beginalignat2
 -0,1 &< E &&< 0,1 \
 6,3 &< S &&< 6,4 \
6,3-0,1 &<S+E&&< 6,4+0,1 \
 -0,1 &< I &&< 6,5 
endalignat
enddocument

With array:

documentclassarticle
usepackageamsmath,array

begindocument

beginequation*
renewcommandarraystretch1.2 % like in cases
setlengtharraycolsep0pt % use math spacings
beginarray r< c >l 
 -0,1 < & E & < 0,1 \
 6,3 < & S & < 6,4 \
6,3-0,1 < & S+E & < 6,4+0,1 \
 6,2 < & I & < 6,5
endarray
endequation*

enddocument

align family environments, by nature, produce right/left alignments of successive columns. Thus they are not easily adapted to achieve the center-alignment of the middle column that was requested.

Also, in order to not treat the comma as punctuation, one can either enclose each comma in its own braces , (yuck!), or use the cmac macro provided to use active commas to fix the problem...Just make sure you don't use commas in another context while cmac is active.

With tabstackengine...

documentclassarticle
usepackagetabstackengine
letsvcm,

catcode`,=active %
gdef,svcm

newcommandcmaccatcode`,=active 
TABstackMath
begindocument
[
 TABbinary
 cmac
 setstacktabulargap0pt
 tabularCenterstackrcl
 -0,1 <& E &< 0,1 \
 6,3 <& S &< 6,4 \
6,3-0,1 <&S+E&< 6,4+0,1 \
 6,2 <& I &< 6,5
]
enddocument

If one wants more vertical spacing between the lines, simply add setstackgapL1.2baselineskip to the mix:

Another solution with the eqparbox package:

documentclassarticle
usepackageamsmath
usepackageeqparbox
newcommandeqmathbox[2][M]eqmakebox[#1]$displaystyle#2$

begindocument

beginalign*
 -0,1 &< eqmathboxE < 0,1 \
 6,3 &< eqmathboxS < 6,4 \
6,3-0,1 & <eqmathboxS+E < 6,4+0,1 \
 6,2 & < eqmathboxI < 6,5
endalign*

enddocument