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[16G] Given nonempty sets \(I,A\), a sequence with indexes in \(I\) and taking values in \(A\) is a function \(a:Iβ A\); this though is usually written by the notation \(( a_ n )_{ n β I}\)Β . To denote the codomain, the notation \( ( a_ n )_{ n β I}β A\)Β is also employed. In this text, in most cases, we will have that \(I=β\), and in this case we will simply write \((a_{n})\).