| AKT : A periodical-publication is published regularly, such as once every week. Strictly speaking, the noun 'periodical' is used by librarians to refer to things published at intervals of greater than a day. We use the phase periodical-publication to include newspapers and other daily publications, since they share many bibliographic features. The periodicity indicates how often the publication comes out. Note that this is a duration, rather than a time interval. A time interval indicates a specific time interval on the time continuum, so we need to model periodicity as a time quantity
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