Аннотация:We consider the Cox–Ingersoll–Ross process that satisfies the stochastic
differential equation dX_t = aX_tdt + σ√X_tdBH_t driven by a fractional Brownian motion BH_t with the Hurst index exceeding 2/3. We show that
the Cox–Ingersoll–Ross process coincides with the square of the fractional Ornstein–
Uhlenbeck process up to the first return to zero. Based on this observation, we
consider the square of the fractional Ornstein–Uhlenbeck process with an arbitrary
Hurst index and prove that it satisfies the above stochastic differential equation up
to the first return to zero if t0√XsdBHs is understood as the pathwise Stratonovich
integral. Then a natural question arises about the first visit to zero of the fractional
Cox–Ingersoll–Ross process which coincides with the first visit to zero of the fractional Ornstein–Uhlenbeck process. Since the latter process is Gaussian, we use the
bounds for the distributions of Gaussian processes to prove that the probability of a
visit to zero over a finite time equals 1 if a < 0. Otherwise this probability is positive.
We provide an upper bound for this probability.