We study mixed site-bond directed percolation on 2D and 3D lattices by using
time-dependent simulations. Our results are compared with rigorous bounds
recently obtained by Liggett and by Katori and Tsukahara. The critical
fractions psitecβ and pbondcβ of sites and bonds are extremely well
approximated by a relationship reported earlier for isotropic percolation,
(logpsitecβ/logpsitecββ+logpbondcβ/logpbondcββ=1), where psitecββ and pbondcββ are the critical fractions in
pure site and bond directed percolation.Comment: 10 pages, figures available on request from [email protected]