To compute (2), we need

where is the Kronecker-Delta. Using the nabla operator and (40), we can compress (39):

where is the Hessian of the output . Since the sums over in (40) need to be computed only once (the results are reusable for all ), can be computed in time. The product of the Hessian and a vector can be computed in time (see next section). With constant number of output units,

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