In this paper, we present an overview of the recent developments of functional quantization of stochastic processes, with an emphasis on the quadratic case. Functional quantization is a way to approximate a process, viewed as a Hilbertvalued random variable, using a nearest neighbour projection on a finite codebook. A special emphasis is made on the computational aspects and the numerical applications, in particular the pricing of some path-dependent European options.
IntroductionFunctional quantization is a way to discretize the path space of a stochastic process. It has been extensively investigated since the early 2000's by several authors (see among others [29], [31], [12], [9], [30], etc). It first appeared as a natural extension of the Optimal Vector Quantization theory of (finitedimensional) random vectors which finds its origin in the early 1950's for signal processing (see [15] or [17]).Let us consider a Hilbertian setting. One considers a random vector X defined on a probability space (Ω, A, P) taking its values in a separable Hilbert space (H, (.|.) H ) (equipped with its natural Borel σ-algebra) and satisfying E|X| 2 < +∞. When H is an Euclidean space (R d ), one speaks about Vector Quantization. When H is an infinite dimensional space like L 2 T := L 2 ([0, T ], dt) (endowed with the usual Hilbertian norm |f | L 2 T := (T 0 f 2 (t)dt) 1 2 ) one speaks of functional quantization (denoted L 2 T from now on). A (bi-measurable) stochastic process (X t ) t∈[0,T ] defined on (Ω, A, P) satisfying |X(ω)| L 2 T < +∞ P(dω)-a.s. can always be seen, once possibly modified on a P-negligible set, as an L 2 T -valued random variable. Although we will focus on the Hilbertian framework, other choices are possible for H, in particular some more general Banach settings like L p ([0, T ], dt) or C([0, T ], R) spaces.This paper is organized as follows: in Sections 2 we introduce quadratic quantization in a Hilbertian setting. In Section 3, we focus on optimal quantization, including some extensions to non quadratic quantization. Section 4 is