Yan Xiong Wei Wu Xidai Kang Chao Zhang Training Pi-Sigma Network by Online Gradient Algorithm with Penalty for Small Weight Update. 3356-3368 2007 19 Neural Computation 12 http://dx.doi.org/10.1162/neco.2007.19.12.3356 db/journals/neco/neco19.html#XiongWKZ07