Heavyside step function

The quantity H(x) is a continuous function defined as the relaxation spectrum and is often used in polymer literature. (Note this term should not be confused with the Heavyside step function used earlier to represent a step input.)... 

Considering only the cases where the Heavyside unit step functions in (3.69) differ from zero (i.e., the nontrivial case where r overlaps in the y direction with the reference element), it is observed that (3.82) and (3.83) can never be true unless the test element is in contact with one (or more) of the reference array elements. Since this is not the case, the first term in (3.80) may be dropped. 

The quantity H(t) is the Heavyside or unit step function (See appendix A) and is defined to be. 

A function closely related to the 5-function is the so called 0-function or step function, also known as the Heavyside function ... 

The ESA sets in with a step at time zero modeled by the Heavyside function... 

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