This is a function made only for fun.
It uses Quadrature rules based on interpolating functions with 100 subintervals and 5 decimals points.
It creates a function based on the number, and calculate its definite integral from -1 to 0 and from 0 to 1. Then it compares if the left definite integral equals the right definite integral. If so, returns true.
npm run testDiscreteIntegral's calculate: O(intervals).
isEven: O(intervals) + O(1).