Free apps with ads on iOS or android. Consulting- similar to freelancing without needing to build or maintain anything but relationships, can consist of advising people on what solutions exist and then refusing to implement them.
Rental property and the classical passive income routes like dividend bearing stocks or stuff like lendingclub.com.
You could get a real estate license and sell real estate. You're a normal person, you have normal person options outside of work/freelancing.
Specifically for development, you could also learn enough patent law to go for relevant patents but that'll probably cost more than it earns.
You could offer your services to another company to help them hire developers. Review their code and design interviews and such. But those are pretty much all varieties of 'freelance,' so it depends on what about 'freelance' work you're hesitant to take on.
Tl:dr- In order; graph theory, linear algebra, number theory, and multi-criteria decision analysis. Game theory, set theory, nonlinearity, computational geometry a bonus.
Let's say you're learning some new algorithms. Your list might look something like the below list[1]. I'll add in brackets the sort of math you learn about by learning the algo if you get lost down the wiki-rabbit-hole. If you want to solve other similar problems, more of that sort of math will help.
2) Dynamic programming [self-similarity, finite subdivision rules, nonlinear equations]: Standard dynamic programming problems such as Rod Cutting, Knapsack, Matrix chain multiplication etc.
3) Number theory [Number Theory]: Modular arithmetic, Fermat’s theorem, Chinese remainder theorem(CRT), Euclidian method for GCD, Logarithmic Exponentiation, Sieve of Eratosthenes, Euler’s totient function.
3) Greedy [goal programming, simplex algorithm, multi-criteria decision analysis]: Standard problems such as Activity selection.
4) Search techniques [linear/vector algebra[2]]: Binary search, Ternary search and Meet in the middle.
5) Data structures (Basic) [graph theory]: Stacks, Queues, Trees and Heaps.
6) Data structures (Advanced)[more graph theory]: Trie, Segment trees, Fenwick tree or Binary indexed tree(BIT), Disjoint data structures.
7) Strings [set theory]: Knuth Morris Pratt(KMP), Z algorithm, Suffix arrays/Suffix trees. These are bit advanced algorithms.
8) Computational geometry [computational geometry]: Graham-Scan for convex hull, Line sweep.
9) Game theory [game theory]: Basic principles of Nim game, Grundy numbers, Sprague-Grundy theorem.
Rental property and the classical passive income routes like dividend bearing stocks or stuff like lendingclub.com.
You could get a real estate license and sell real estate. You're a normal person, you have normal person options outside of work/freelancing.
Specifically for development, you could also learn enough patent law to go for relevant patents but that'll probably cost more than it earns.
You could offer your services to another company to help them hire developers. Review their code and design interviews and such. But those are pretty much all varieties of 'freelance,' so it depends on what about 'freelance' work you're hesitant to take on.