You probably know all of this already, but here goes...
Write C′w=Tw+sumx<wpx,wTx where px,winumathbbZ[u]. Now, the other basis can be defined by applying the involutive automorphism b:mathcalHntomathcalHn, given by b(Tw)=Tw and b(u)=−u−1.
Define Cw:=b(C′w).
Since, b commutes with the bar involution, this basis is bar invariant as well.
Explicitly, Cw=Tw+sumx<w(−1)ell(w)+ell(x)barpx,wTx.
So Cw=barP−1PC′w which seems hard to compute in general.
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