Mathematics
Orthogonal Polynomials and Their Recurrence Relations
Quick fact
Every family of orthogonal polynomials—like Legendre, Chebyshev, or Hermite—can be generated by a simple three-term recurrence relation, which makes them incredibly efficient to compute and use in numerical algorithms.
Why this is interesting
You know that two lines are perpendicular if they meet at a right angle. But what does it mean for two functions to be perpendicular? And why would that be useful?