Consider the two sentences below:
Relationship between the element number and its denominator.
Relationship between the element number and the denominator.
They both are grammatically correct, however, can mean two different things. You should be able to select the apt sentence based on your mathematical function.
The first sentence uses an 'its' which shows a sense of ownership. From the first sentence, one can deduce that the 'element number' is a numerator. As far as the second sentence is concerned, the element in discussion need not be a numerator.
For example, let us assume that x is a number and y is another, non-zero number.
The first sentence can be mathematically interpreted as the relationship between 'x' and 'y' from the equation:
x / y
However the second sentence can be used to convey the relationship between 'x' and 'y' in an equation like:
x + (z / y)
So, choose wisely, based on your expression.