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The multiplicative subgroup of all elements in the product of the multiplicative groups 
 whose
absolute value is 1 at all but finitely many 
, where 
 is a number Field and 
 a Place.
See also Adéle
References
Knapp, A. W.  ``Group Representations and Harmonic Analysis, Part II.''  Not. Amer. Math. Soc. 43, 537-549, 1996.