This seems a trivial change but because of alteration of
basic symmetry it is one
of the few percolation variants that really give different critical
There is as yet no explicit solution, but there are many
rigorous results and
the various quantities are extremely well behaved. I personally
believe it is solvable.
Reggeon Field Theory falls in the same universality class.
Lets orient the square lattice in directions north, south, east and west.
if the ant of the
previous topic can only walk north and east and never south
or west it would be more difficult to find a connected path.
The critical exponents would also change.
There are immediate applications to percolation of forest fires in a wind.
A beautiful application of directed percolation to galaxy formation is
in the project of Liron Glezer.