Question 1081079
.
It is for the first time in my life I see this term "primary interval".


Surely, the correct term MUST BE "period".


When I asked GOOGLE for the term "primary interval", if gave me ~7000 results, which is practically ZERO.


When I asked GOOGLE for the term "period" to compare, if gave me ~1,200,000,000 results, which is normal quantity, as I expected.



<U>Conclusion and Diagnosis</U>. The person who invented the term "primary interval" as a substitution 
for/instead of the term "period" is UNFAMILIAR with the commonly accepted mathematical terminology.



OK, if and when such unusual term is introduced, then, at least, it must be defined in the condition.