A basic assumption in current halo occupation model is that the properties of a galaxy depend only on the mass of its dark matter halo .
An important consequence of this is that the segregation of the galaxy population by large-scale environment is entirely due to the environmental dependence of the halo population .
In this paper we use such a model to predict how the galaxy luminosity function depends on large-scale environment .
The latter is represented by the density contrast ( \delta ) averaged over a spherical volume of radius R = 8 \ > h ^ { -1 } { Mpc } .
The model predicts that the Schechter function is a good approximation to the luminosity functions of galaxies brighter than \sim 10 ^ { 9 } h ^ { -2 } \ > { L _ { \odot } } ( b _ { j } -band ) in virtually all environments .
The characteristic luminosity , L ^ { \star } , increases moderately with \delta .
The faint-end slope , \alpha , on the other hand , is quite independent of \delta .
However , when splitting the galaxy population into early and late types , it is found that for late-types \alpha is virtually constant , whereas for early-types \alpha increases from \sim - 0.3 in underdense regions ( \delta \sim - 0.5 ) to \sim - 0.8 in highly overdense regions with \delta \sim 10 .
The luminosity function at L _ { b _ { j } } < 10 ^ { 9 } h ^ { -2 } \ > { L _ { \odot } } is significantly steeper than the extrapolation of the Schechter function that fits the brighter galaxies .
This steepening is more significant for early-types and in low-density environments .
The model also predicts that the luminosity density and mass density are closely correlated .
The relation between the two is monotonic but highly non-linear .
This suggests that one can use the luminosity density , averaged over a large volume , to rank the mass density .
This , in turn , allows the environmental effects predicted here to be tested by observations .