Training a single sigmoidal neuron is hard

  • Authors:
  • Jiří Sima

  • Affiliations:
  • Institute of Computer Science, Academy of Sciences of the Czech Republic, P.O. Box 5, 18207 Prague 8, Czech Republic

  • Venue:
  • Neural Computation
  • Year:
  • 2002

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Abstract

We first present a brief survey of hardness results for training feedforward neural networks. These results are then completed by the proof that the simplest architecture containing only a single neuron that applies a sigmoidal activation function σ: R → [α, β], satisfying certain natural axioms (e.g., the standard (logistic) sigmoid or saturated-linear function), to the weighted sum of n inputs is hard to train. In particular, the problem of finding the weights of such a unit that minimize the quadratic training error within (β - α)2 or its average (over a training set) within 5(β - α)2/(12n) of its infimum proves to be NP-hard. Hence, the well-known backpropagation learning algorithm appears not to be efficient even for one neuron, which has negative consequences in constructive learning.