# Translations:Wavelets - book/6/en

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For the record, all of the digital filters that we consider (unless otherwise stated) fall under the category of *linear time-invariant filters*. Note that filter is called *linear* if it satisfies the additive property and the multiplicative property. Let a given input yield a given output. A filter is called *time-invariant* if the same input delayed (or advanced) by a given amount yields the same output delayed (or advanced) by the same amount (Robinson and Silvia, 1978^{[1]}).

- ↑ Robinson, E. A., and M. T. Silvia, 1978, Digital signal processing and time series analysis: Holden Day, Inc.