Prove or disprove: Let $G$ be a group of order 595, then G has a cyclic subgroup of order 35.
What I think is that the statement is true. If $|G|=595=5\times 7\times 17$, then since 5 and 7 divides $|G|$, there exists elements $x,y\in G$ such that $|x|=5, |y|=7$, so $A=\langle x\rangle, B=\langle y\rangle$. By computations we see that number of Sylows 5-subgroups is 1, so there is a unique Sylows 5-subgroup of order 5, which will be $A$, since all Sylows 5-subgroup are conjugate. So, $A$ is normal subgroup of $G$, and $B$ is a subgroup of $G$, Thus, $AB$ is a subgroup of $G$, and
$$|AB|=\frac{|A||B|}{|A\cap B|}=\frac{5\times 7}{1}=35$$ because 5 and 7 are relatively prime.
S0, $|AB|=35$, and $AB\simeq \mathbb Z_{5}\times \mathbb Z_{7} \simeq \mathbb Z_{35}$ since 5 and 7 are relatively prime.
Note: A well known reasult: Any group of order 35 is cyclic, by the Fundamental Theorem of Finite Abelian Groups.
Is my argument correct in general (there are many details, using Sylows Theorem, Cauch Theorem,..)!?
\timesor\cdot(the latter is a raised dot); instead of<and>for subgroup-generated, use\langleand\rangle; the latter are from the parenthesis family, so they have proper spacing around them, whereas<and>are operators so the space around them is off. – Arturo Magidin Aug 16 '11 at 19:47