  Although the AdS_5xS^5 worldsheet action is not quadratic, some features of
the pure spinor formalism are simpler in an AdS_5xS^5 background than in a flat
background. The BRST operator acts geometrically, the left and right-moving
pure spinor ghosts can be treated as complex conjugates, the zero mode measure
factor is trivial, and the b ghost does not require non-minimal fields.
  Furthermore, a topological version of the AdS_5xS^5 action with the same
worldsheet variables and BRST operator can be constructed by gauge-fixing a G/G
principal chiral model where G=PSU(2,2|4). This topological model is argued to
describe the zero radius limit that is dual to free N=4 super-Yang-Mills and
can also be interpreted as an "unbroken phase" of superstring theory.
