## What are examples of good toy models in mathematics?

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A comment on another question reminded me of this old post of Terence Tao's about toy models. I really like the idea of using toy models of a difficult object to understand it better, but I don't know of too many examples since I don't see too many people talk about them. What examples are common in your field, or what examples do you personally think are very revealing? Here's what I've got so far, starting with Terence Tao's example. Feel free to modify any of these examples if I'm not stating them correctly and to elaborate on them in answers if you want.

• Fp[t] is a toy model for Z.
• Fp[[t]] is a toy model for Zp.
• Simplicial complexes are a toy model for topological spaces.
• Z/nZ is a toy model for Z (for the purposes of additive number theory).
• The DFT is a toy model for the Fourier transform on the circle.

Which properties of the original objects carry over to your toy model, and which don't? As usual, stick to one example per post.

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Are simplicial complexes really toy models for topological spaces? While the category of simplicial complexes has numerous problems, they are mostly confined to the morphisms -- most spaces of interest are at least homotopy equivalent to simplicial complexes. – Andy Putman Oct 20 2009 at 3:37
Well, I wouldn't really know; feel free to edit that one or write an answer. – Qiaochu Yuan Oct 20 2009 at 3:42
Not sure if this is the right place to ask, but when you say F<sub>p</sub>[t] is a toy model for Z I guess you are referring to the general philosophy that problems over function fields are easier to deal with than those over number fields. Can someone actually elaborate on this analogy between number fields and function fields? I'm not sure where I can find information about this. Ring of integers being Dedekind, finite residue field, RH over function fields easier to deal with, anything else? – Ho Chung Siu Oct 20 2009 at 4:07
I have the same question; I think it should probably be asked separately. – Qiaochu Yuan Oct 20 2009 at 4:14
I think the category of CW complexes (c.f. Hatcher's book) is a toy model for topological spaces. – John Mangual Dec 27 2009 at 15:38

One can think of toric varieties as "toy examples" of algebraic varieties. A lot can be said about them via combinatorial data, but they definitely are special (they are always rational varieties, for example).

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Topological quantum field theories are toy models for real quantum field theories.

This is a funny example though, as TQFTs later became "real" again, when it was discovered that they describe the physics of the Fractional Quantum Hall Effect. Toy models sometimes pay off big!

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The best examples I've come up with come from rational homotopy theory--commutative differential graded Q-algebras as a toy model for spaces and chain complexes of Q-vector spaces as a toy model for spectra--though really this is an instance of "toy examples" because we can build actual spaces/spectra corresponding to these algebraic data. It feels a bit like a toy model, though, I guess because those spaces aren't very geometric.

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2x2 matrices are a toy model for general square matrices.

"If an assertion about matrices is false, there is usually a 2x2 matrix that reveals this." -- Olga Tausky-Todd

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 Similarly, the matrix algebras M_n(K) are nice toy models for general central simple algebras (along with the Hamiltonians in the Quaternionic case). – Ben Linowitz Jan 20 2010 at 16:21

Shift spaces are examples of topological dynamical systems, but they also serve as a toy model for topological dynamical systems.

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Your first example of Fp[t] being a toy model for Z is a facet of the well known (to number theorists) analogy between Q and Fp(t). This analogy extends fabulously to a more general connection between the arithmetic of number fields, i.e. finite extensions of Q, and of function fields, i.e. finite extensions of Fp(t).

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Perhaps this doesn't count as a toy model, rather a toy example. A nice basic example for GIT is n points in CP1 under the action of SL(2,C). A lot of the usual elements of the theory look nice in this picture. For example, the Hilbert Mumford criterion shows that a collection of n points is semi-stable iff all points have multiplicity less or equal to n/2 whilst a collection of n points is stable iff all points have multiplicity strictly less than n/2.

It's also a nice example of the equivalence between symplectic reduction and the GIT quotient. If you fix a Fubini-Study metric on CP1 and look at the action of the corresponding SU(2) you can ask for a moment map. Thinking of CP1 as a coadjoint orbit in su(2)*, the moment map takes n points to their centre of mass. Now the equivalence of symplectic and GIT quotients says that, provided we don't have two points each of multiplicity n/2, you can move n points in CP1 by an element of SL(2,C) so that their centre of mass is zero if and only if all multiplicities are strictly less than n/2. (The case when two points each have multiplicity n/2 is special because of the additional C* stabiliser.) In one direction this is obvious, but in the other I think this is a neat non-trivial statement (at least when n is large).

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One can think of Grothendieck ring of varieties as a toy model for motives.

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Relations are a toy model for linear maps. In fact, they can be thought of as matrices over the boolean semi-ring.

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Derived categories of finite dimensional hereditary algebras for other derived categories. (Though under good circumstances other derived categories can turn out to be equivalent to derived categories of f.d. hereditary algebras.)

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2-bridge knots (rational knots) are a toy model for knots. If you think something should be right, first test it on 2-bridge knots. This happens so much in knot theory that it's not even funny.

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