The dual of an abelian group $A$ is defined to be the group $\text{Hom}(A,\mathbb Z)$ of homomorphisms to the infinite cyclic group. As usual with such dualities, there's a canonical homomorphism from $A$ to its double dual
$$
A\to A^{**}:a\mapsto(h\mapsto h(a)).
$$
If this is an isomorphism, $A$ is said to be reflexive. 

**Question:** Are all free abelian groups reflexive?

**Answer:** Yes if and only if there are no measurable cardinals.