I'd have to say... Linear Algebra & SQL. SQL is something that is a permanent journey for me. It is a domain-specific language, so the nature of its use depend

I'd have to say... Linear Algebra & SQL. SQL is something that is a permanent journey for me. It is a domain-specific language, so the nature of its use depends almost entirely on how the domain was modeled in the first place. Much of my interest has been drawn towards the modeling aspects and how we can arrive at schemas that business experts can tolerate. Linear algebra properly clicked for me when I started getting into writing my own 3d rendering software. Systems of linear equations are absolutely everywhere. I didn't really tie together their use in control theory, DSP and graphics until I was forced to learn them "for real" to achieve my project goals.