Nonparametric adaptive estimation for pure jump L´ evy processes. Fixed sample step versus high frequency data. Fabienne Comte(1) Joint works with V. Genon-Catalot(1) (1) MAP5, UMR 8145, Universit´e Paris Descartes. Introduction: L´evy processes for modelling purposes in many areas: Eberlein and Keller (1995), Barndorff-Nielsen and Shephard (2001), Cont and Tankov (2004). Bertoin (1996) or Sato (1999): comprehensive study for these processes. Distribution of a L´evy process: specified by its characteristic triple (drift, Gaussian component and L´ evy measure.) Rather than by the distribution of its independent increments (intractable) ⇒ standard parametric approach by likelihood methods difficult task. ⇒ nonparametric methods. L´evy measure interesting to estimate because specifies the jumps behavior. Nonparametric estimation of the L´ evy measure Recent contributions: Basawa and Brockwell (1982): non decreasing L´evy processes and observations of jumps with size larger than some positive ε, or discrete observations with fixed sampling interval. Nonparametric estimators of a distribution function linked with the L´evy measure. See also Hoffmann and Krell (2007). Figueroa-L´ opez and Houdr´ e (2006): a continuous-time observation of a general L´evy process and study penalized projection estimators of the L´evy density. Neumann and Reiss (2009). real-valued L´evy processes of pure jump type, i.e. without drift and Gaussian component. Assumption: the L´evy measure admits a density n(x) on R. Context: Process discretely observed with sampling interval ∆. Let (Lt ) denote the underlying L´evy process and (Zk∆ = Lk∆ − L(k−1)∆ , k = 1, . . . , n) the observed random variables (i.i.d.) Characteristic function of L∆ = Z1∆ is given by: Z ψ∆ (u) = E(exp iuZ1∆ ) = exp (∆ (eiux − 1)n(x)dx) R where the unknown function is the L´evy density n(x). Z ′ ψ∆ (u) = iE(Z1∆ exp iuZ1∆ ) = i∆ eiux xn(x)dx ψ∆ (u). R (1) Thus, if R R |x|n(x)dx < ∞, we get with g(x) = xn(x): Z ′ ψ∆ (u) ∗ iux g (u) = e g(x)dx = −i . ∆ψ∆ (u) (2) Nonparametric estimation strategy using empirical estimators of the characteristic functions and Fourier inversion. See also Watteel-Kulperger (2003) and Neumann-Reiss (2009). ⇒ Estimate g ∗ (u) by using empirical counterparts of ψ∆ (u) ′ and ψ∆ (u) = iE(Z1 eiuZ1 ) only. ⇒ Problem of estimating g = deconvolution-type problem. But: Problem of deconvolution from (2) is not standard Both the numerator and the denominator are estimated ⇒ Deconvolution in presence of unknown error density. + Have to be estimated from the same data. Estimator of ψ∆ (u) is not a simple empirical counterpart. Truncated version analogous to the one used in Neumann (1997) and Neumann and Reiss (2007). Technical assumptions up to now: R (H1) |x|n(x)dx < ∞. R R (H2(p)) For p integer, R |x|p−1 |g(x)|dx < ∞. (H3) The function g belongs to L2 (R). Our estimation procedure is based on the i.i.d. r.v. Z∆ i = Li∆ − L(i−1)∆ , i = 1, . . . , n, (3) with common characteristic function ψ∆ (u). Moments of Z1∆ linked with g: Proposition 1 Let p ≥ 1 integer. Under (H2)(p), E(|Z1∆ |p ) < ∞. R k−1 Moreover, setting, for k = 1, . . . p, Mk = R x g(x)dx, we have E(Z1∆ ) = ∆M1 , E[(Z1∆ )2 ] = ∆M2 + ∆2 M1 , and more generally, l E[(Z∆ 1 ) ] = ∆ Ml + o(∆) for all l = 1, . . . , p. Control of ψ∆ . ∀x ∈ R, cψ (1 + x2 )−∆β/2 ≤ |ψ∆ (x)| ≤ Cψ (1 + x2 )−∆β/2 , (H4) for some given constants cψ , Cψ and β ≥ 0. Also considered in Neumann and Reiss (2007). For the adaptive version of our estimator, we need additional assumptions for g: (H5) and (H6) There exists some positive a such that R ∗ |g (x)|2 (1 + x2 )a dx < +∞, R x2 g 2 (x)dx < +∞. Independent assumptions for ψ∆ and g: there may be no relation at all between these two functions. Examples. Compound Poisson processes. Lt = Nt X Yi , Yi i.i.d. with density f i=1 (Yi ) independent of Nt , Nt ∼ Poisson(c). P(L∆ = 0) = e−c∆ n(x) = cf (x). e−2c∆ ≤ |ψ∆ (u)| ≤ 1. The L´ evy Gamma process. Lt ∼ Γ(βt, α) n(x) = βx−1 e−αx 1(x > 0). β∆ α ψ∆ (u) = . α − iu Bilateral Gamma process. K¨ uchler and Tappe (2008). (1) (2) (1) (2) Lt = Lt − Lt , Lt and Lt independent and L´evy-Gamma. Parameters (β ′ , α′ ; β, α). Special case: β ′ = β and α′ = α. Variance-Gamma (Madan and Seneta, 1990). Lt = WZt , (W ) Brownian motion independent of Z and Z L´evy-Gamma. • Bilateral Gamma. n(x) = x−1 g(x) ′ g(x) = β ′ e−α x 1(x > 0) − βe−α|x| 1(x > 0). ψ∆ (u) = α α − iu β∆ ′ α α′ + iu β ′ ∆ . Notations ∗ ∗ u the Fourier transform of the function u: u (y) = kuk2 = < u, v >= Z Z R eiyx u(x)dx, |u(x)|2 dx, u(x)v(x)dx with zz = |z|2 . For any integrable and square-integrable functions u, u1 , u2 , (u∗ )∗ (x) = 2πu(−x) and hu1 , u2 i = (2π)−1 hu∗1 , u∗2 i. (4) Definition of the estimator. ′ ψ θ∆ (x) ∗ ∆ (x) g (x) = −i = , ∆ψ∆ (x) ∆ψ∆ (x) (5) with ixZ1∆ ψ∆ (x) = E(e ), θ∆ (x) = ′ −iψ∆ (x) = ∆ ixZ1∆ E(Z1 e ). n n X X ∆ 1 1 ixZk ∆ ixZ∆ ˆ ˆ ψ∆ (x) = e , θ∆ (x) = Zk e k . n n k=1 k=1 Although |ψ∆ (x)| > 0 for all x, this is not true for ψˆ∆ . As Neumann (1997) and Neumann and Reiss (2007), truncate 1/ψˆ∆ 1 1 = 1I|ψˆ∆ (x)|>κψ n−1/2 . ˜ ˆ ψ∆ (x) ψ∆ (x) 1 g ˆm (x) = 2π Z πm −πm e −ixu θˆ∆ (u) du. ˜ ∆ψ∆ (u) (6) Projection formulation. √ sin(πx) and ϕm,j (x) = mϕ(mx − j). ϕ(x) = πx ϕ∗m,j (x) Sm eixj/m = √ 1I[−πm,πm] (x). m (7) = Span{ϕm,j , j ∈ Z} = {h ∈ L2 (R), supp(h∗ ) ⊂ [−mπ, mπ]}. {ϕm,j }j∈Z orthonormal basis (Sm )m∈Mn the collection of linear spaces, Mn = {1, . . . , mn } and mn ≤ n is the maximal admissible value of m, subject to constraints to be given later. Consider gm orthogonal projection of g on Sm Z X gm = am,j (g)ϕm,j with am,j (g) = ϕm,j (x)g(x)dx = hϕm,j , gi. R j∈Z and hϕm,j , gi = 1 ∗ hϕm,j , g∗ i. 2π The estimator can be defined by: g ˆm = X ˆ am,j ϕm,j , with ˆ am,j j∈Z or a ˆm,j 1 = 2πn∆ 1 = 2π∆ Z n X k=1 Z∆ k Z e ixZ∆ k ϕ∗m,j (−x) θˆ∆ (x) dx. ˜ ψ∆ (x) ϕ∗m,j (−x) dx, ˜ ψ∆ (x) Let t ∈ Sm of the collection (Sm )m∈Mn , and define Z n ∗ X 1 1 t (−x) 2 ∆ ixZ∆ γn (t) = ktk − dx, Zk e k ˜ π∆ n ψ∆ (x) (8) k=1 Consider γn (t) as an approximation of the theoretical contrast Z n ∗ X 1 1 th 2 ∆ ixZk∆ t (−x) dx, γn (t) = ktk − Zk e π∆ n ψ∆ (x) k=1 E(γnth (t)) = ktk2 − 2hg, ti = kt − gk2 − kgk2 minimal for t = g. We have also g ˆm = Argmint∈Sm γn (t). (9) We define: Φψ (m) = Z πm −πm (analogy with deconvolution setting) dx , |ψ∆ (x)|2 Proposition 2 Under (H1)-(H2)(4)-(H3), for all m: 4 E1/2 [(Z∆ 1 ) ]Φψ (m) E(kg − g ˆm k ) ≤ kg − gm k + K . 2 n∆ 2 where K is a constant. 2 (10) Discussion about the rates kg − gm k2 = Z |x|≥πm |g ∗ (x)|2 dx. Suppose that g belongs to the Sobolev class Z S(a, L) = {f, |f ∗ (x)|2 (x2 + 1)a dx ≤ L}. Then, the bias term satisfies kg − gm k2 = O(m−2a ). Under (H4), the bound of the variance term satisfies R πm 2β∆+1 2 dx/|ψ (x)| ∆ m −πm =O . n∆ n∆ The optimal choice for m is O((n∆)1/(2β∆+2a+1 ) and the resulting rate for the risk is (n∆)−2a/(2β∆+2a+1 ). Sampling interval ∆ explicitly appears in the exponent of the rate. ⇒ for positive β, the rate is worse for large ∆ than for small ∆. Thus we can state the following corollary of Proposition 2: Corollary 1 Under assumptions (H1)-(H2(4))-(H3)-(H5), then E(kˆ gm −gk2 ) = O(n∆)−2a/(2β∆+2a+1 ) when m = O((n∆)1/(2β∆+2a+1 ). Study of the adaptive estimator We have to select an adequate value of m. pen(m) = κ(1 + E[(Z1∆ )2 ]/∆) Φψ (m) . n∆ (11) We set m ˆ = arg min {γn (ˆ gm ) + pen(m)} , m∈Mn and study first the “risk” of gˆm ˆ. Theorem 1 Assume that assumptions (H1)-(H2)(8)-(H3)-(H7) hold. Then 2 E(kˆ gm ˆ − gk ) ≤ C inf m∈Mn where K is a constant. 2 ln (n) kg − gm k2 + pen(m) + K , n∆ Assumption on the collection of models Mn = {1, . . . , mn }, mn ≤ n: (H7) ∃ε, 0 < ε < 1, m2β∆ ≤ Cn1−ε , n where C is a fixed constant and β is defined by (H4). For instance, Assumption (H7) is fulfilled if: 1. pen(mn ) ≤ C. In such a case, we have mn ≤ C(n∆)1/(2β∆+1) . 2. ∆ is small enough to ensure 2β∆ < 1. Take Mn = {1, . . . , n}. (H7) = problem because depends on the unknown β But concrete implementation requires the knowledge of mn . Analogous deconvolution with unknown error density. In the compound Poisson model, β = 0 and nothing is needed. Otherwise one should at least know if ψ∆ is in a class of polynomial decay. Estimator of β (see e.g. Diggle et Hall (1993))? To get an estimator, we replace the theoretical penalty by: ! R πm n ˜∆ (x)|2 X dx/| ψ 1 −πm ∆ 2 pd en(m) = κ′ 1 + . (Z ) i n∆2 n i=1 In that case we can prove: Theorem 2 Assume that assumptions (H1)-(H2)(8)-(H3)-(H7) hold and let g˜ = gˆm be the estimator defined with b b b m b = arg minm∈M (γn (ˆ gm ) + pd en(m)). Then n 2 E(k˜ g − gk ) ≤ C inf m∈Mn 2 kg − gm k + pen(m) + 2 ′ ln (n) K∆ n ′ where K∆ is a constant depending on ∆ (and on fixed quantities but not on n). If g belongs to the Sobolev ball S(a, L), and under (H4), the rate is automatically of order O((n∆)−2a/(2β∆+2a+1) ). High frequency context: ∆ small, n∆ large, ψ∆ (u) = 1 + O(∆). Define g˘m = arg min γ¯n (t) (n) t∈Sm where γn (t) = ktk2 − ¯ 1 πn∆ n Z X Zk exp (iuZk )t∗ (−u)du. (12) k=1 Then E(¯ γn (t)) = ktk2 − 2ht, gi− 2 2π Z (ψ∆ (u) − 1)g∗ (u)t∗ (−u)du. (13) As ψ∆ (u) − 1 = O(∆), the last term above is negligible if ∆ is small, and ktk2 − 2ht, gi = kt − gk2 − kgk2 . Deterministic residual term. Z 1 (ψ∆ (u) − 1)g ∗ (u)t∗ (−u)du Rn (t) = 2π New risk bound: Proposition 3 Assume that (H2) holds, then 2 2 E(kg − g ˘m k ) ≤ 3kg − gm k + ⇒ ∆ small, n∆ large. 2[E(Z21 )/∆] m + C∆2 n∆ Z πm −πm u2 |g∗ (u))|2 du. Z S(a, L) = g ∈ (L1 ∩ L)2 (R), (1 + u2 )a |g ∗ (u)|2 du ≤ L . kg − gm k2 = Z |u|≤πm |g ∗ (u)|2 du ≤ L(πm)−2a . Proposition 4 Assume that (H2) holds, that M2 < +∞. Then if g belongs to S(a, L) and if n → +∞, ∆ → 0 and n∆2 ≤ 1, we have E(kg − g˘m k2 ) ≤ O((n∆)−2a/(2a+1) ). If a ≥ 1, then the constraints n∆2 ≤ 1 becomes n∆3 = o(1). m ˘ = arg min (¯ γn (˘ gm ) + pen(m)) ˘ m∈Mn with pen(m) ˘ =κ Denote by 1 n∆ n X i=1 Z2i ! m . n∆ penth (m) = E(pen(m)) = κ(E(Z12 )/∆) m . n∆ Theorem 3 Assume that (H1)-H2(8)-(H3) are fulfilled and that n is large and ∆ is small, n∆ tends to infinity when n tends to infinity. Then 2 2 E(kg − g ˘m kg − gm k + penth (m) ˘ k ) ≤ C inf m∈Mn Z C∆2 πmn 2 ∗ + u |g (u))|2 du. 2π −πmn
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