I run epsilon-svr on about 1000 samples (RBF kernel), and then calculate the in-sample training errors.

The hyperparameters are found via a search, ranging from 1e-5 to 1e2. model->eps is a couple orders of magnitude lesser than model->p.

From my understanding, the KKT conditions are:

1. sv_coef[i] == 0         <==>   abs( predict(prob->x[i]) - y[i] )  <= model->param.p,
2. 0 < abs(sv_coef[i]) < C <==>   abs( predict(prob->x[i]) - y[i] )  == model->param.p  (or extremely close)
3. abs(sv_coef[i]) == C    <==>   abs( predict(prob->x[i]) - y[i] ) >= model->param.p
4. abs(sv_coef[i]) <= C

Conditions 1,2,3 break for me over many of the training vectors, e.g.:

Violated KKT conditions for RemainingSet: Weight/Cost: 0/700, label: 1.00288, predicted: 1.0025, Error/Eps: 0.000372128/0.0001, computed violation: 0.000272128

As printed, a non-support vector does not like inside the epsilon tube. The algorithm does not hit max iterations, so it claims that it has converged to the global optimum.

Does LibSVM output a broken system in epsilon-SVR mode? Is the error caused by numerical stability? How would one go about to stabilize an SVM with violated KKT conditions?


1 Answer 1


LibSVM uses 1-indexed counting for a certain structure, which must be dealt with - the svm_model.sv_indices array. When used correctly, all 4 KKT conditions hold in my case.


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