  We use 2D hydrodynamic simulations of viscous disks to examine whether
dynamically-interacting multiple giant planets can explain the large gaps
(spanning over one order of magnitude in radius) inferred for the transitional
and pre-transitional disks around T Tauri stars. In the absence of inner disk
dust depletion, we find that it requires three to four giant planets to open up
large enough gaps to be consistent with inferences from spectral energy
distributions, because the gap width is limited by the tendency of the planets
to be driven together into 2:1 resonances. With very strong tidal torques
and/or rapid planetary accretion, fewer planets can also generate a large
cavity interior to the locally formed gap(s) by preventing outer disk material
from moving in. In these cases, however, the reduction of surface density
produces a corresponding reduction in the inner disk accretion rate onto the
star; this makes it difficult to explain the observed accretion rates of the
pre/transitional disks. We find that even with four planets in disks,
additional substantial dust depletion is required to explain observed disk
gaps/holes. Substantial dust settling and growth, with consequent significant
reductions in optical depths, is inferred for typical T Tauri disks in any
case, and an earlier history of dust growth is consistent with the hypothesis
that pre/transitional disks are explained by the presence of giant planets. We
conclude that the depths and widths of gaps, and disk accretion rates in
pre/transitional disks cannot be reproduced by a planet-induced gap opening
scenario alone. Significant dust depletion is also required within the
gaps/holes. Order of magnitude estimates suggest the mass of small dust
particles (<1 micron) relative to the gas must be depleted to 1e-5 -- 1e-2 of
the interstellar medium value, implying a very efficient mechanism of small
dust removal or dust growth.
