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Russo–Vallois integral : ウィキペディア英語版
Russo–Vallois integral
In mathematical analysis, the Russo–Vallois integral is an extension to stochastic processes of the classical Riemann–Stieltjes integral
:\int f \, dg=\int fg' \, ds
for suitable functions f and g. The idea is to replace the derivative g' by the difference quotient
:g(s+\varepsilon)-g(s)\over\varepsilon and to pull the limit out of the integral. In addition one changes the type of convergence.
==Definitions==
Definition: A sequence H_n of stochastic processes converges uniformly on compact sets in probability to a process H,
:H=\text\lim_H_n,
if, for every \varepsilon>0 and T>0,
:\lim_\mathbb(\sup_|H_n(t)-H(t)|>\varepsilon)=0.
One sets:
:I^-(\varepsilon,t,f,dg)=\int_0^tf(s)(g(s+\varepsilon)-g(s))\,ds
:I^+(\varepsilon,t,f,dg)=\int_0^t f(s)(g(s)-g(s-\varepsilon)) \, ds
and
:()_\varepsilon (t)=\int_0^t(f(s+\varepsilon)-f(s))(g(s+\varepsilon)-g(s))\,ds.
Definition: The forward integral is defined as the ucp-limit of
:I^-: \int_0^t fd^-g=\text\lim_I^-(\varepsilon,t,f,dg).
Definition: The backward integral is defined as the ucp-limit of
:I^+: \int_0^t f \, d^+g = \text\lim_I^+(\varepsilon,t,f,dg).
Definition: The generalized bracket is defined as the ucp-limit of
:()_\varepsilon: ()_\varepsilon=\text\lim_()_\varepsilon (t).
For continuous semimartingales X,Y and a cadlag function H, the Russo–Vallois integral coincidences with the usual Ito integral:
:\int_0^t H_s \, dX_s=\int_0^t H \, d^-X.
In this case the generalised bracket is equal to the classical covariation. In the special case, this means that the process
:():=() \,
is equal to the quadratic variation process.
Also for the Russo-Vallois Integral an Ito formula holds: If X is a continuous semimartingale and
:f\in C_2(\mathbb),
then
:f(X_t)=f(X_0)+\int_0^t f'(X_s) \, dX_s + \int_0^t f''(X_s) \, d()_s.
By a duality result of Triebel one can provide optimal classes of Besov spaces, where the Russo–Vallois integral can be defined. The norm in the Besov space
:B_^\lambda(\mathbb^N)
is given by
:||f||_^\lambda=||f||_ + \left(\int_0^\infty )^q \, dh\right)^
with the well known modification for q=\infty. Then the following theorem holds:
Theorem: Suppose
:f\in B_^\lambda,
:g\in B_^,
:1/p+1/p'=1\text1/q+1/q'=1.
Then the Russo–Vallois integral
:\int f \, dg
exists and for some constant c one has
:\left| \int f \, dg \right| \leq c ||f||_^\alpha ||g||_^.
Notice that in this case the Russo–Vallois integral coincides with the Riemann–Stieltjes integral and with the Young integral for functions with finite p-variation.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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