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Equation 8 · How Mechanistic Interpretability Research Is Actually Done

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a^\hat a

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a^\hat a

Symbol hat a

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Sparse dictionary learning is the field’s answer, and it is a second and different act of extraction rather than a departure from the first: a sparse autoencoder is trained on the very same activation vectors the probe was reading, learning an overcomplete basis in which each vector is reconstructed as a sparse combination of dictionary elements. Writing a for the activation, z for its sparse code and a^\hat a for the reconstruction,

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