Let pt(x,⋅) denote the Wiener
measure in an abstract Wiener space (H,B) with variance parameter t > 0 and mean
x in B. It is shown that if f ∈ L2pt(x,⋅),t > 0 and x are fixed, then the function ptf
defined by ptf(x + h) = ∫
Bf(y)pt(x + h,dy) for h in H is infinitely Gross differentiable
at x. The first two derivatives are given by (Dptf(x),h) = t−1 ∫
Bf(y)(h,y −x)pt(x,dy)
and (D2ptf(x)k,h) = t−1 ∫
Bf(y){t−1(h,y −x)(k,y −x) − (h,k)}pt(x,dy), where h and
k are in H. Moreover, D2ptf(x) is a Hilbert-Schmidt operator and ∥D2ptf(x)∥2 ≦
t−1{∫
B|f(y)|2pt(x,dy)}1∕2. An application to Uhlenbeck-Ornstein process is also
given.
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