Try this problem from TIFR 2013 problem 5 based on Non-Cyclic Subgroup of \(\mathbb{R}\).
Question: TIFR 2013 problem 5
True/False?
All non-trivial proper subgroups of \((\mathbb{R},+)\) are cyclic.
Hint: What subgroups comes to our mind immediately?
Discussion: \((\mathbb{Q},+)\) is a subgroup of \((\mathbb{R},+)\). Is \((\mathbb{Q},+)\) a cyclic group?
Suppose \((\mathbb{Q},+)\) is cyclic. Then there exists a generator say \(\frac{a}{b}\). Note that, we are only allowed to use addition (and subtraction) to create \(\mathbb{Q})\
Therefore, we can create $$ \frac{a}{b}+\frac{a}{b}+...+\frac{a}{b}=n\frac{a}{b}=\frac{na}{b} $$
Also, we can create $$ (-\frac{a}{b})+(-\frac{a}{b})+...+(-\frac{a}{b})=n(-\frac{a}{b})=-\frac{na}{b} $$
Notice that we can increase the magnitude of the numerator, but not the magnitude of the denominator.
For example, we cannot create \(\frac{a}{2b}\) using \(\frac{a}{b}\) and the binary operation +.
Therefore, \((\mathbb{Q},+)\) is not cyclic.
Remark: There is one result which states that subgroups of \((\mathbb{R},+)\) are either cyclic or dense. Notice that although \((\mathbb{Q},+)\) is not cyclic it is dense.
