Trivial math with a relatively inaccessible notation. The notation is why most people seem to have a hard time recognizing it as math I think, people have tunnel-vision and think of "math" as the stuff they learned in primary school.

Non-trivial theoretical basis, reduced to trivial math for implementation.

JPEG and wavelet compression in practice is surprisingly simple math. But the theory that explains why and how it works is far from trivial.