Edit index.md — Applications of Math

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# applications of math

many people say math is useless. "fine, addition and multiplication are useful, but I learned those early. why is calculus useful?"

this wiki is my answer. math applies to everything — not just in the obvious "you need it for engineering" way, but in a deeper way: advanced math gives you ways of thinking that restructure how you see the world. the biggest use of advanced math is the ability to organize and structure things.

the wiki has three layers, from the most immediately practical to the most profoundly structural.

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## layer 1: the immediate and basic

the math you use every day without calling it math. these seem trivial, but they're everywhere — and getting them wrong has real consequences.

- [[counting-and-measurement|counting and measurement]] — all of science, commerce, and engineering starts with measuring things. precision vs accuracy. units as a type system. measurement theory.
- [[arithmetic-everywhere|arithmetic everywhere]] — addition, subtraction, multiplication, division. so embedded in daily life that we forget they're math. compound interest. dosage calculations. cooking ratios.
- [[ordering-and-comparison|ordering and comparison]] — sorting, ranking, prioritizing. total and partial orders. arrow's impossibility theorem. elo ratings. pareto optimality.
- [[probability-in-daily-life|probability in daily life]] — "should I bring an umbrella?" bayesian reasoning. base rate neglect. the monty hall problem. expected value. risk assessment.
- [[patterns-and-estimation|patterns and estimation]] — fermi estimation. order-of-magnitude reasoning. mental math. pattern recognition and its failures.

## layer 2: the STEM foundation

the classic "math is the language of science" argument. true and important, but well-known — so these pages are concise.

- [[physics|physics]] — the original applied math. newton's laws as differential equations. quantum mechanics as linear algebra. the unreasonable effectiveness of mathematics.
- [[computer-science|computer science]] — boolean algebra → circuits → computers. algorithms and complexity. cryptography. information theory. machine learning.
- [[engineering-and-modeling|engineering and modeling]] — math modeling competitions. monte carlo simulation. optimization. spectral methods. "all models are wrong but some are useful."
- [[biology-and-medicine|biology and medicine]] — population dynamics. epidemiology. genetics. EEG signal processing. medical statistics.

## layer 3: the profound and structural

this is the most interesting layer — and the one most people miss. abstract math doesn't just solve problems. it gives you **ways of thinking about fundamental patterns**. each branch provides a lens.

- [[the-organizational-lens|the organizational lens]] — my core thesis. advanced math's biggest use is as an organizational tool for seeing structure in everyday things.
- [[calculus-as-thinking|calculus as thinking]] — not computing derivatives. thinking about change, accumulation, limits, and continuity as universal patterns.
- [[linear-algebra-as-thinking|linear algebra as thinking]] — every idea as a vector. every process as a matrix. eigenvalues as stability. semantic space and word embeddings.
- [[set-theory-as-thinking|set theory as thinking]] — unions, intersections, MECE partitions. binary classification as set membership. the discipline of precise categorization.
- [[multivariable-calculus-as-thinking|multivariable calculus as thinking]] — gradient (which direction?), divergence (source or sink?), curl (circulation?). gradient descent and optimization.
- [[abstraction-as-power|abstraction as power]] — why abstraction is useful. the abstraction ladder. category theory. the trade-off between generality and specificity.
- [[topology-as-thinking|topology as thinking]] — invariants. what stays the same under deformation. connectedness. fundamental groups. topological data analysis.
- [[symmetry-and-groups|symmetry and groups]] — group theory. noether's theorem. the rubik's cube. music theory. why symmetric solutions tend to be elegant.