  We derive an expression for the non-Gaussian cosmic-microwave-background
(CMB) statistic $I_l^3$ defined recently by Ferreira, Magueijo, and G\'orski in
terms of the slow-roll-inflation parameters $\epsilon$ and $\eta$. This result
shows that a nonzero value of $I_l^3$ in COBE would rule out single-field
slow-roll inflation. A sharp change in the slope of the inflaton potential
could increase the predicted value of $I_l^3$, but not significantly. This
further suggests that it will be difficult to account for such a detection in
multiple-field models in which density perturbations are produced by quantum
fluctuations in the scalar field driving inflation. An Appendix shows how to
evaluate an integral that is needed in our calculation as well as in more
general calculations of CMB bispectra.
