The subjet multiplicity in quark and gluon jets

We calculate the mean number of subjets in quark and gluon jets in the final state of e^+e^- annihilation. Since `quark' and `gluon' jets are scheme-dependent objects, we stress the importance of using the same definition as in experimental analyses. We define the jets using the k_perp algorithm at a coarse scale y_1, and the subjets using a finer scale y_0, in the same algorithm. Gluon jets are anti-tagged by the presence of heavy quarks in both other jets. Our result is exact to leading order in alpha_s, and resums leading and next-to-leading logarithmic terms in the ratio y_1/y_0 to all orders in alpha_s.