  We show that some $q$-series such as universal mock theta functions are
linear sums of theta quotients and mock Jacobi forms of weight 1/2, which
become holomorphic parts of real analytic modular forms when they are
restricted to torsion points and multiplied by suitable powers of $q$. And we
prove that certain linear sums of $q$-series are weakly holomorphic modular
forms of weight 1/2 due to annihilation of mock Jacobi forms or completion by
mock Jacobi forms. As an application, we obtain a relation between the rank and
crank of a partition.
