This is particularly relevant in electronics, where the oscilloscope's time origin is set to match the wave.
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A series adds the terms of a sequence in their specified order. Its infinite-sum notation is defined through finite partial sums:when the displayed limit of a sequence exists as a finite number. Otherwise the series is not a convergent series in the ordinary sense. Writing an infinite list of additions does not by itself assign it a finite sum. Cesàro summation is a different prescription, with its own definition.
Two positive series make the distinction concrete. For , the individual terms tend to zero but the harmonic series has unbounded partial sums. For , partial fraction decomposition reveals a telescoping series whose partial sums tend to one. The useful question is how much all the remaining terms can contribute together, rather than only how small the next term looks.