Ghost number 2 should correspond to the physical vertex operators:
| (51) |
We have only studied them in flat space.
As we will explain, there is a puzzle
even in flat space.
It is more convenient to study the homology
, instead of the cohomology.
The relation between them is the Poincare duality:
| (52) |
The homology has a very straightforward elementary description. It consists of the
elements of the form:
| (53) |