My czech book on English in mathematics has a section about choosing the right article. It says:
Use the indefinite article when you are referring to each element of a class. For example: Mersenne primes are related to perfect numbers.
Use the definite article when you are referring to all elements of a class. For example: The harmonics numbers are the numbers H^n for \alpha > 1 defined by the formula...
I am deeply confused since I do not get the difference between the examples. I do not get the difference between each element and all elements neither.