Strongly Typical Sequences (strong Typicality, Letter Typicality)
If a sequence x1, ..., xn is drawn from some specified joint distribution defined over a finite or an infinite alphabet, then the strongly typical set, Aε,strong(n) is defined as the set of sequences which satisfy
where is the number of occurrences of a specific symbol in the sequence.
It can be shown that strongly typical sequences are also weakly typical (with a different constant ε), and hence the name. The two forms, however, are not equivalent. Strong typicality is often easier to work with in proving theorems for memoryless channels. However, as is apparent from the definition, this form of typicality is only defined for random variables having finite support.
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