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Structured support vector machine : ウィキペディア英語版
Structured support vector machine
The structured support vector machine is a machine learning algorithm that generalizes the Support Vector Machine (SVM) classifier. Whereas the SVM classifier supports binary classification, multiclass classification and regression, the structured SVM allows training of a classifier for general structured output labels.
As an example, a sample instance might be a natural language sentence, and the output label is an annotated parse tree. Training a classifier consists of showing pairs of correct sample and output label pairs. After training, the structured SVM model allows one to predict for new sample instances the corresponding output label; that is, given a natural language sentence, the classifier can produce the most likely parse tree.
==Training==
For a set of \ell training instances (\boldsymbol_n,y_n) \in \mathcal\times\mathcal, n=1,\dots,\ell from a sample space \mathcal and label space \mathcal, the structured SVM minimizes the following regularized risk function.
:\underset \quad \|\boldsymbol\|^2 + C \sum_^
\underset \left(\Delta(y_n,y) + \boldsymbol'\Psi(\boldsymbol_n,y) - \boldsymbol'\Psi(\boldsymbol_n,y_n)\right)
The function is convex in \boldsymbol because the maximum of a set of affine functions is convex. The function \Delta: \mathcal \times \mathcal \to \mathbb_+ measures a distance in label space and is an arbitrary function (not necessarily a metric) satisfying \Delta(y,z) \geq 0 and \Delta(y,y)=0 \;\; \forall y,z \in \mathcal. The function \Psi: \mathcal \times \mathcal \to \mathbb^d is a feature function, extracting some feature vector from a given sample and label. The design of this function depends very much on the application.
Because the regularized risk function above is non-differentiable, it is often reformulated in terms of a quadratic program by introducing one slack variable \xi_n for each sample, each representing the value of the maximum. The standard structured SVM primal formulation is given as follows.
:\begin
\underset} & \|\boldsymbol\|^2 + C \sum_^ \xi_n\\
\textrm & \boldsymbol' \Psi(\boldsymbol_n,y_n) - \boldsymbol' \Psi(\boldsymbol_n,y) + \xi_n \geq \Delta(y_n,y),\qquad n=1,\dots,\ell,\quad \forall y \in \mathcal
\end

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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