recursive, (edited ) 
I was thinking of making a list of types of software that most people think is "too difficult" and they leave to "the experts", e.g.:
- geometric modeling kernel
- integrated circuit floorplanning
- cryptography
- compiler backends
- database query planners
- electromagnetic simulation (antennas, transmission lines, microstrip design, etc)
...Oh, it's all applied math.
(As an aside, I just want to note that there's some really difficult work to be done in accessibility and UX too.)
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