Frequency

== Aperiodic frequency ==
'''Aperiodic frequency''' is the [[rate (mathematics)|rate]] of incidence or occurrence of non-[[Periodic function|cyclic]] phenomena, including random processes such as [[radioactive decay]]. It is expressed with the [[unit of measurement|unit]] of [[reciprocal second]] (s<sup>−1</sup>)<ref>{{Cite book |title=Mechatronic Systems, Sensors, and Actuators: Fundamentals and Modeling |last=Lombardi |first=Michael A. |publisher=CRC Press |year=2007 |isbn=9781420009002 |editor-last=Bishop |editor-first=Robert H. |location=Austin |language=en|chapter=Fundamentals of Time and Frequency}}</ref> or, in the case of radioactivity, [[becquerels]].<ref>{{cite report | last1=Newell | first1=David B | last2=Tiesinga | first2=Eite | title=The international system of units (SI) | publisher=National Institute of Standards and Technology | publication-place=Gaithersburg, MD | year=2019 | doi=10.6028/nist.sp.330-2019 | url = https://nvlpubs.nist.gov/nistpubs/SpecialPublications/NIST.SP.330-2019.pdf}} sub§2.3.4, Table 4.</ref>

It is defined as a [[rate (mathematics)|rate]], ''f'' = ''N''/Δ''t'', involving the [[number of entities]] counted or the number of [[Event (philosophy)|event]]s happened (''N'') during a given [[Time|time duration]] (Δ''t'');{{cn|date=July 2023}} it is a [[physical quantity]] of type [[temporal rate]].