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=== Functional linear models with functional responses ===
For a functional response <math>Y(\cdot)</math> with ___domain <math>\mathcal{T}</math> and a functional covariate <math>X(\cdot)</math> with ___domain <math>\mathcal{S}</math>, two FLMs regressing <math>Y(\cdot)</math> on <math>X(\cdot)</math> have been considered<ref name=wang:16/><ref>Ramsay and [[Bernard Silverman|Silverman]] (2005). ''Functional data analysis'', 2nd ed., New York : Springer, [[Special:BookSources/038740080X|ISBN 0-387-40080-X]].</ref>. One of these two models is of the form
{{NumBlk|::|<math display="block">Y(t) = \beta_0(t) + \int_{\mathcal{S}} \beta(s,t) X^c(s)ds + \epsilon(t),\ \text{for}\ t\in\mathcal{T},</math>|{{EquationRef|4}}}}
where <math>X^c(\cdot) = X(\cdot) - \mathbb{E}(X(\cdot))</math> is still the centered functional covariate, <math>\beta_0(\cdot)</math> and <math>\beta(\cdot,\cdot)</math> are coefficient functions, and <math>\epsilon(\cdot)</math> is usually assumed to be a random process with mean zero and finite variance. In this case, at any given time <math>t\in\mathcal{T}</math>, the value of <math>Y</math>, i.e., <math>Y(t)</math>, depends on the entire trajectory of <math>X</math>. Model ({{EquationNote|4}}), for any given time <math>t</math>, is an extension of [[Multivariate linear regression|multivariate linear regression]] with the inner product in Euclidean space replaced by that in <math>L^2</math>. An estimating equation motivated by multivariate linear regression is
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