Abstract
This contribution surveys the main characteristics of two stochastic processes that generalize the fractional Brownian motion: the multifractional Brownian motion and the multifractional processes with random exponent. A special emphasis will be devoted to the meaning and to the applications that they can have in finance. If fractional Brownian motion is by now very well-known and studied as a model of the price dynamics, multifractional processes are yet widely unknown in the field of quantitative finance, mainly because of their nonstationarity. Nonetheless, in spite of their complex structure, such processes deserve consideration for their capability to seize the stylized facts that most of the current models cannot account for. In addition, their functional parameter provides an insightful and parsimonious interpretation of the market mechanism, and is able to unify in a single model two opposite approaches such as the theory of efficient markets and the behavioral finance.
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Notes
- 1.
Remind that the random process \(Y_{t,\tau}\), \(\tau>t\), is a stochastic discount factor (or pricing kernel) if (a) \(\mathbb{P}(Y_{t,t}=1,Y_{t,\tau}>0)=1\), and (b) \(S_t=\mathbb{E}^\mathbb{P}_t[Y_{t,\tau}S_\tau]\).
- 2.
The definition of Hölder exponent will be given in paragraph 2; here, we just recall that it measures the degree of irregularity of the graph of a function.
- 3.
The distinction between these two ways of measuring the local regularity is not just a mathematical detail, since-as proved in (Ayache 2013)-while the trajectories of an arbitrary centered Gaussian process share the same global (uniform) as well as local Hölder regularity, the same property does not hold with respect to the pointwise Hölder regularity.
- 4.
We remind that the stochastic process \(\left\{X(t,\omega)\right\}_{t\in T}\) is said self-similar of parameter H if for any \(a>0\) and \(t\in T\), it is \(\{X(at,\,\omega )\}\overset{d}{\mathop{=}}\,\,\{{{a}^{H}}X(t,\,\omega )\}\), where the equality holds for the finite-dimensional distributions of \(\left\{X(t,\omega)\right\}\).
- 5.
- 6.
The fBm can be also defined in terms of random wavelet series as
$$\bar{B}(x,H) = \sum_{j=-\infty}^{+\infty} \sum_{k \in \mathbb{Z}} 2^{-jH} \epsilon_{j,k} \left(\Psi(2^jx-k,H) -\Psi(-k,H) \right)$$(10)where \(\left\{\epsilon_{j,k}\right\}_{(j,k)\in \mathbb{Z}^2}\) is a sequence of independent \(N(0,1)\) random variables; \(\Psi \in C^\infty(\mathbb{R}\times(0,1))\) is well-localized in the first variable and uniformly localized in H, which means that, for all \((n,p)\in \mathbb{N}^2\), \(\sup \left\{(1+|x|)^p|(\partial_x^{(n)}\Psi)(x,H)|:(x,H)\in \mathbb{R}\times(0,1)\right\}<+\infty.\) This representation-which allows to define the MPRE (see below) also when its random exponent depends on the Brownian measure-is almost surely uniformly convergent in \((x,H)\) on each compact subset of \(\mathbb{R}\times(0,1)\).
- 7.
The corrections mainly concerned the revision of classical definitions of arbitrage and self-financing condition (Hu and Oksendal 2003; Elliott and Van Der Hoek 2003); the regularization of the weighting kernel of the fBm (Rogers 1997; Cheridito 2001), the introduction of transaction costs (Guasoni 2006 or of delays in transaction times Cheridito 2003).
- 8.
Remind that the function h(t) is Hölderian of order β on each compact interval \(J \subset \mathbb{R}\) if, for each \(t,s \in J\) and for \(c>0\), it holds \(|h(t)-h(s)| \leq c|t-s|^\beta\), where \(\beta> \max_{t \in J}h(t)\).
- 9.
Indeed, (Ayache and Taqqu 2005) provide sufficient conditions for a multifractional process with random functional parameter to be self-similar (in the sense of its marginal distributions) or to have stationary increments.
- 10.
To avoid ambiguity, when necessary, we write explicitly ω for the stochastic process.
- 11.
Basically, with regard to the process X sampled at N times, the generalized quadratic variation is defined as \(V_N=\sum_{p=0}^{N-2}\left(X\left(\frac{p+2}{N}\right)-2X\left(\frac{p+1}{N}\right)+X\left(\frac{p}{N}\right)\right)^2\); the variation serves to define the estimator \(\hat{h}_N=\frac{1}{2}\left(1-\frac{lnV_N}{lnN}\right)\) which, under some assumptions on the function h(t), satisfies \(lim_{N\rightarrow+\infty}\hat{h}_N=inf_{t\in(0,1)}h(t)\) almost surely. The result stated for the infimum provides the way to estimate h itself at any point \(t\in(0,1)\); in fact choosing a proper \((\epsilon,N)\)-neighborhood of t one can calculate the generalized quadratic variation \(V_{\epsilon,N}\) and hence the estimator \(\hat{h}_{\epsilon,N}\), that satisfies \(lim_{N\rightarrow+\infty}\hat{h}_{\epsilon,N}=inf_{|s-t|<\epsilon}h(s)\) (a.s). Letting ϵ tend to zero one gets an estimate of h(t).
- 12.
The variance in (23) follows from assuming a smooth h(t). In fact, from (16) it follows:
$$\begin{aligned} Var\left(X_{t}-X_{s}\right)&=\mathbb{E}\left(X_{t}-X_{s}\right)^{2} -\left(\mathbb{E}(X_{t}-X_{s})\right) ^{2}= \mathbb{E}\left(X_{t}^{2}+X_{s}^{2}-2X_{t}X_{s}\right) = \\ &= t^{2h(t)}+s^{2h(s)}-D(h(t),h(s))\left(t^{h(t)+h(s)}+s^{h(t)+h(s)}-|t-s|^{h(t)+h(s)}\right)\\ &= t^{h(t)}(t^{h(t)}-D(h(t),h(s))t^{h(s)})+s^{h(s)}(s^{h(s)}-D(h(t),h(s))s^{h(t)}) + \\ & \hspace{6cm} + D(H_{t},H_{s})|t-s|^{h(t)+h(s)}.\end{aligned}$$Since \(\lim_{|h(t)-h(s)| \rightarrow 0}D(h(t),h(s))=1\), whenever \(h(t)\approx h(s)\), one has \(Var\left(X_{t}-X_{s}\right) \approx |t-s|^{2h(t)}\). Assuming that the mBm is sampled in discrete time over n points with \(Var(X_{n}-X_{0})=K^{2}\) entails therefore
$$\begin{aligned}Var\left(X_{\frac{t+q}{n-1}}-X_{\frac{t}{n-1}}\right) \cong K^{2}\left | \frac{t+q}{n-1}-\frac{t}{n-1} \right | ^{2h\left(\frac{t}{n-1}\right)} = K^{2}\left(\frac{q}{n-1}\right) ^{2h\left(\frac{t}{n-1}\right)}\end{aligned}$$that the variance in (23)
- 13.
For \(h \not= \frac{1}{2}\), the variance of \(h_{\delta,q,n,1}^k(t)\) is hard to deduce because of the term σ2 that appears in the variance of (30).
- 14.
Function L is slowly varying at infinity if \(\lim_{t \to \infty} \frac{L(\alpha t)}{L(t)}=1 \text{ for some} \alpha \in \mathbb{R^+}\)
- 15.
To avoid confusion with the notation, here we indicate the order of the absolute moment by s (instead of k used for \(h_{\delta,q,n,K}^{k}(t)\)).
- 16.
Using the two-sided Lilliefors test at different significance levels, Bianchi and Pianese 2008 show that, consistently with the mBm/MPRE model, the empirical significance values of three main stock indexes converge to the nominal ones as δ decreases. Although this suggests to maintain δ as short as possible in order to ensure the normality of data, a trade-off problem arises because the estimator’s variance increases as the length of the window decreases. The discussion of this effect leads the authors in (Bianchi and Pianese 2008) to set \(\delta=30\).
- 17.
This assumption is not restrictive, since it has just been shown that k = 2 minimizes the estimator’s variance (see Remark 14).
- 18.
Likewise the notation already introduced, from now on the subscript will refer to the discrete time sampling.
- 19.
Using a time changing δ (see Remark 14) produces very similar results, hence to save the homogeneity of the results we have chosen a fixed window of proper length.
- 20.
The Jarque-Bera test quantifies the distance from the Gaussian distribution in terms of skewness and kurtosis and then computes a single p value using the sum of these discrepancies.
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Bianchi, S., Pianese, A. (2015). Asset Price Modeling: From Fractional to Multifractional Processes. In: Bensoussan, A., Guegan, D., Tapiero, C. (eds) Future Perspectives in Risk Models and Finance. International Series in Operations Research & Management Science, vol 211. Springer, Cham. https://doi.org/10.1007/978-3-319-07524-2_7
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