Xu, L. (1993). "Least Mean Square Error Reconstruction Principle for Self-Organizing Neural-Nets," Neural Networks, 6(5), 627-648.

Xu, L. (1994). "Theories of Unsupervised Learning: PCA and its Nonlinear Extensions," IEEE International Conference on Neural Networks, (Orlando 1994), vol. XXX, XXXpage numbers XXX, IEEE Press.