This week I worked on solving trigonometric inequalities.

Trigonometric inequalities Link to heading

The primary univariate inequality solver - solve_univariate_inequality,
depends upon the results of solve in order to solve the given inequality. Taking a cue from PR#10022 on incorporating solveset for inequalities, I worked on developing an approach for replacing the use of solve with solveset in solving inequalities.

Idea

Convert Set-type output from solveset to list objects similar to that returned by solve.

The most striking difference between both the APIs is the uniform Set output returned by solveset. Hence, the prime concern while transitioning from solve to solveset should be handling the various type of solutions.

Here are a few implementation ideas on the same:

  • FiniteSet : finite number of solutions
    Using the list constructor on these type of objects works extremely well.

  • ImageSet : infinite number of solutions
    This is generally the case with trigonometric functions as most of them are periodic in nature. We need to limit the number of solutions to be finite.
    For this, I intend to use the a periodic interval: [0, period] as the basis for filtering the solution set. This returns a simplified FiniteSet of solutions which can be used to solve inequalities in a restricted interval. Following which we can generalise the output over the entire domain of the function.

    A major issue here is the representation of the final solution set.
    For example:

    In []: solveset(cos(x)<0, x, domain=S.Reals)
⎡         π           3⋅π         ⎤
⎢2*n*pi + ─, 2*n*pi + ───  | n ∊ ℤ⎥
⎣         2            2          ⎦

    Currently, we do not have a Set object for representing this.
    For this, we need to implement an Indexed Union : Union(X(a) for a in I)

    We can symbolically represent the above solution as BigUnion(Interval(2*n*pi + pi/2, 2*n*pi + 3*pi/2), n, S.Intgers).

After thoughts

Also, this week my PR#11277 on periodicity got merged finally. I have updated the corresponding PR#11141 which has been stalled for some time now. Hopefully, it will get merged soon.

The next week I will devote my time to the implementation part of solving inequalities.