Name a field of math

The rarest possible answer — almost no player finds it.

• Tropical geometry— Named for Brazil-based Imre Simon, it flattens curves into piecewise lines.

Genuinely obscure. Most players never think of it.

• Abelian group theory
• Absolute geometry
• Abstract analytic number theory
• Abstract differential geometry
• Abstract harmonic analysis
• Additive combinatorics
• Additive number theory
• Affine differential geometry
• Affine geometry
• Ahlfors theory
• Algebraic analysis
• Algebraic combinatorics
• Algebraic graph theory
• Algebraic K-theory
• Algebraic logic
• Algebraic number theory
• Algebraic statistics
• Algebraic topology
• Algorithmic information theory
• Anabelian geometry
• Analysis on metric spaces
• Analytic combinatorics
• Analytic number theory
• Analytic theory of L-functions
• Applied probability
• Approximation theory
• Arakelov geometry
• Arithmetic combinatorics
• Arithmetic dynamics
• Arithmetic geometry
• Arithmetic topology
• Asymptotic analysis
• Auslander–Reiten theory
• Automata theory
• Bayesian statistics
• Bifurcation theory
• Birational geometry
• C*-algebra theory
• Calculus of moving surfaces
• Calculus of variations
• Catastrophe theory
• Categorical logic
• Character theory
• Chemical graph theory
• Chemical reaction network theory
• Class field theory
• Clifford analysis
• Clifford theory
• Cobordism theory
• Coding theory
• Cohomology theory
• Combinatorial commutative algebra
• Combinatorial design theory
• Combinatorial game theory
• Combinatorial group theory
• Combinatorial optimization
• Combinatory logic
• Commutative algebra
• Complex algebraic geometry
• Complex analytic geometry
• Complex differential geometry
• Complex dynamics
• Complex geometry
• Computability theory
• Computable analysis
• Computable model theory
• Computational algebraic geometry
• Computational complexity theory
• Computational geometry
• Computational group theory
• Computational mathematics
• Computational number theory
• Computational statistics
• Computational synthetic geometry
• Computational topology
• Computer algebra
• Conformal geometry
• Constructive analysis
• Constructive function theory
• Constructive mathematics
• Constructive set theory
• Contact geometry
• Control theory
• Convex analysis
• Convex geometry
• Convex optimization
• CR geometry
• Credibility theory
• Decision analysis
• Decision theory
• Derived noncommutative algebraic geometry
• Descriptive geometry
• Descriptive set theory
• Design of experiments
• Difference algebra
• Difference equations
• Differential algebra
• Differential algebraic geometry
• Differential Galois theory
• Differential topology
• Diffiety theory
• Diophantine approximation
• Diophantine geometry
• Discrepancy theory
• Discrete differential geometry
• Discrete exterior calculus
• Discrete geometry
• Discrete Morse theory
• Discrete tomography
• Distance geometry
• Distribution theory (generalized functions)
• Domain theory
• Donaldson theory
• Dynamical systems
• Effective descriptive set theory
• Elementary number theory
• Elimination theory
• Elliptic geometry
• Engineering mathematics
• Enumerative combinatorics
• Enumerative geometry
• Equivariant noncommutative algebraic geometry
• Ergodic Ramsey theory
• Ergodic theory
• Euler calculus
• Evolutionary game theory
• Experimental mathematics
• Exterior calculus
• Extraordinary cohomology theory
• Extremal combinatorics
• Extremal graph theory
• Extreme value theory
• Field theory
• Finite geometry
• Finite model theory
• Finsler geometry
• First-order logic
• Formal language theory
• Foundations of mathematics
• Fractional calculus
• Fractional dynamics
• Fredholm theory
• Functional analysis
• Functional calculus
• Functional data analysis
• Functional equations
• Fuzzy logic
• Fuzzy measure theory
• Fuzzy set theory
• Galois cohomology
• Galois geometry
• Galois theory
• Gauge theory
• General topology
• Generalized trigonometry
• Geometric algebra
• Geometric analysis
• Geometric calculus
• Geometric combinatorics
• Geometric evolution equations
• Geometric function theory
• Geometric graph theory
• Geometric group theory
• Geometric invariant theory
• Geometric measure theory
• Geometric probability
• Geometric topology
• Geometry of numbers
• Geostatistics
• Global analysis
• Global differential geometry
• Gyrotrigonometry
• Hamiltonian dynamics
• Harmonic analysis
• Higher category theory
• Higher-dimensional algebra
• Higher-order logic
• Hodge theory
• Hodge–Arakelov theory
• Homological algebra
• Homology theory
• Homotopical algebra
• Homotopy theory
• Homotopy type theory
• Hopf algebra theory
• Hyperbolic dynamics
• Hyperbolic geometry
• Hyperbolic trigonometry
• Hypercomplex analysis
• Hyperfunction theory
• Idempotent analysis
• Incidence geometry
• Infinitary combinatorics
• Information geometry
• Information-based complexity
• Integer programming
• Integral equations
• Integral geometry
• Integral transforms
• Intersection theory
• Interval analysis
• Intuitionistic logic
• Intuitionistic mathematics
• Intuitionistic type theory
• Invariant theory
• Inventory theory
• Inverse problems
• Inversive geometry
• Inversive ring geometry
• Itô calculus
• Iwasawa theory
• Iwasawa–Tate theory
• Jordan algebra theory
• K-homology
• K-theory
• Kähler geometry
• KK-theory
• Klein geometry
• Kummer theory
• L-theory
• Large deviations theory
• Large sample theory
• Lattice theory
• Lie sphere geometry
• Lie theory
• Limit theory
• Line geometry
• Local algebra
• Local class field theory
• Low-dimensional dynamics
• Low-dimensional topology
• Malliavin calculus
• Markov processes
• Mathematical biology
• Mathematical chemistry
• Mathematical crystallography
• Mathematical economics
• Mathematical epidemiology
• Mathematical geology
• Mathematical linguistics
• Mathematical modeling
• Mathematical oncology
• Mathematical psychology
• Mathematical sociology
• Mathematical statistics
• Matrix theory
• Matroid theory
• Measure theory
• Metamathematics
• Metric geometry
• Microlocal analysis
• Modal logic
• Model theory
• Modular representation theory
• Module theory
• Morse theory
• Multilinear algebra
• Multiple-criteria decision analysis
• Multiplicative number theory
• Multivariate statistics
• Network theory
• Nevanlinna theory
• Nielsen theory
• Non-abelian class field theory
• Non-Archimedean analysis
• Nonassociative algebra
• Noncommutative algebra
• Noncommutative algebraic geometry
• Noncommutative geometry
• Noncommutative harmonic analysis
• Noncommutative topology
• Nonlinear analysis
• Nonlinear programming
• Nonparametric statistics
• Nonstandard analysis
• Nonstandard calculus
• Nonstandard topology
• Numerical algebraic geometry
• Numerical linear algebra
• Operad theory
• Operator algebras
• Operator K-theory
• Operator theory
• Optimal control theory
• Optimal transport
• Orbifold theory
• Order theory
• Ordered geometry
• Oscillation theory
• p-adic analysis
• p-adic dynamics
• p-adic Hodge theory
• Parabolic geometry
• Paraconsistent mathematics
• Parametric statistics
• Partition theory
• Percolation theory
• Permutation group theory
• Perturbation theory
• Picard–Vessiot theory
• Piecewise linear topology
• Pluripotential theory
• Pointless topology
• Poisson geometry
• Polyhedral combinatorics
• Possibility theory
• Potential theory
• Predicative mathematics
• Probabilistic combinatorics
• Probabilistic graph theory
• Probabilistic number theory
• Probabilistic numerics
• Projective differential geometry
• Projective geometry
• Proof theory
• Pseudo-Riemannian geometry
• Quantum algebra
• Quantum calculus
• Quantum group theory
• Quaternionic analysis
• Queueing theory
• Ramsey theory
• Random dynamical systems
• Random matrix theory
• Rational geometry
• Real algebraic geometry
• Real K-theory
• Recreational mathematics
• Regression analysis
• Renewal theory
• Representation theory
• Representation theory of algebras
• Representation theory of groups
• Reverse mathematics
• Ribbon theory
• Riemann surface theory
• Riemannian geometry
• Ring theory
• Rough path theory
• Rough set theory
• Ruin theory
• Scheduling theory
• Scheme theory
• Scientific computing
• Secondary calculus
• Semigroup theory
• Sequential analysis
• Set-theoretic topology
• Several complex variables
• Sheaf theory
• Sieve theory
• Singularity theory
• Smooth dynamics
• Smooth infinitesimal analysis
• Social choice theory
• Special functions
• Spectral geometry
• Spectral graph theory
• Spectral theory
• Spherical geometry
• Spherical trigonometry
• Statistical distribution theory
• Stochastic analysis
• Stochastic control theory
• Stochastic geometry
• Stochastic optimization
• Stochastic processes
• Stratified Morse theory
• Summability theory
• Surgery theory
• Survey sampling
• Survival analysis
• Symbolic dynamics
• Symplectic geometry
• Synthetic differential geometry
• Synthetic geometry
• Systolic geometry
• Tensor calculus
• Theory of computation
• Tiling theory
• Time series analysis
• Time-scale calculus
• Topological combinatorics
• Topological data analysis
• Topological degree theory
• Topological dynamics
• Topological geometry
• Topological graph theory
• Topological group theory
• Topological K-theory
• Topological transformation groups
• Topos theory
• Toric geometry
• Transcendental number theory
• Transformation geometry
• Twisted K-theory
• Type theory
• Umbral calculus
• Uncertainty quantification
• Uncertainty theory
• Universal algebra
• Universal hyperbolic trigonometry
• Utility theory
• Valuation theory
• Variety theory
• Wavelet analysis

Uncommon — only a minority of players land here.

• Abstract algebra
• Actuarial science
• Algebraic geometry
• Analysis
• Analytic geometry
• Applied mathematics
• Biostatistics
• Boolean algebra
• Category theory
• Chaos theory
• Combinatorics
• Complex analysis
• Cryptography
• Differential calculus
• Differential geometry
• Discrete mathematics
• Econometrics— Statistics shaped for economic data, a field in its own right.
• Euclidean geometry
• Finite mathematics
• Fourier analysis
• Fractal geometry
• Game theory
• Graph theory
• Group theory
• Information theory
• Integral calculus
• Knot theory
• Lambda calculus
• Linear programming
• Mathematical finance
• Mathematical logic
• Mathematical physics
• Multivariable calculus
• Non-Euclidean geometry
• Numerical analysis
• Operations research
• Optimization
• Partial differential equations
• Pure mathematics
• Real analysis
• Set theory
• Solid geometry
• Vector calculus

A solid, middling pick shared with plenty of other players.

• Arithmetic
• Differential equations
• Linear algebra
• Number theory
• Probability
• Topology

The most common answer on the board.

• Algebra
• Calculus
• Geometry
• Statistics
• Trigonometry