  General analytical expressions for Double Quantum Nuclear Magnetic Resonance
(NMR) kinetic curves of many-spin I=1/2 systems are derived with an accuracy of
the second cumulant approximation. The expressions obtained exactly describe
the initial part of the kinetic curves and provide a reasonable approximation
up to times of about twice the effective spin-relaxation time. For the case
when the system contains two isolated spins, this result exactly reproduces
known expressions. In the case of polymer melts, the intermolecular magnetic
dipole-dipole interactions significantly influence the time dependence of the
DQ NMR kinetic curves.
