The string $$1101$$ does not belong to the set represented by

How many substrings of different lengths (non-zero) can be formed from a character string of length $$n$$ ?

$$\sum { = \left\{ {a,b} \right\},\,\,} $$ which one of the following sets is not countable.

Which two of the following four regular expressions are equivalent?
(i) $${\left( {00} \right)^ * }\left( {\varepsilon + 0} \right)$$
(ii) $${\left( {00} \right)^ * }$$
(iii) $${0^ * }$$
(iv) $$0\,\,{\left( {00} \right)^ * }$$