# Contents

## Idea

Since generally an equation is the statement of equality $\phi(x) = \psi(y)$ of two functions $\phi$ and $\psi$ of variables $x$ and $y$, so a linear equation is an equation between linear functions.

### In 1-category theory

Typically here a linear function is taken to mean an $R$-linear map over some given ring $R$, hence a homomorphism $\phi : X \to Z$ or $\psi : Y \to Z$ of $R$-modules $X, Y, Z \in R$Mod. If here $Z$ is an $R$-module of rank greater than 1, one also speaks of a system of linear equations.

Specifically if $R = k$ is a field then these are linear maps of $k$-vector spaces and hence in this case a linear equation is a statement of equality of two vectors $\phi(x) = \psi(y)$ in some vector space $Z$ that depend linearly on vectors $x$ in a vector spaces $X$ and $y \in Y$.

Frequently this is considered specifically for the case that $g$ is a constant function, hence just a fixed vector. In this case the linear equation becomes $\phi(x) = g$ for $x \in X$. If moreover $\phi$ here is represented or representable by a matrix this is typically written as

$\phi \cdot\vec x = \vec g \,,$

which is the form that one finds in standard textbooks on linear algebra. If $\vec g = 0$ here this is called a homogeneous linear equation.

But linear equations make sense and are important in the greater generality where $R$ is not necessarily a field, and in fact in contexts more general than that of $R$-modules even. For instance natural isomorphisms between linear functors are a kind of categorification of linear equations.

### In $(\infty,1)$-category theory

Indeed, as discussed at equation, in the formal logic of type theory a an equation as above is a judgement of the form

$x : X , y : Y \vdash (\phi(x) = \psi(y)) : Type$

whose solution space is the dependent sum

$Sol \;\; \coloneqq \vdash \sum_{{x : X} \atop {y : Y}} (\phi(x) = \psi(y)) : Type$

and reading this in fact in homotopy type theory says that $X, Y, Sol$ are homotopy types.

The generalization of a ring $R$ to a homotopy type is an E-∞-ring and that of an $R$-module $X, Y$ is a module spectrum.

Accordingly a linear equation in homotopy(type) homotopy theory is a statement of equivalence between elements of an $R$-module spectrum that depend $R$-linearly on other $R$-module spectra. More precisely, as discussed at equation, the solution space to such an “$\infty$-linear equation” is the homotopy pullback

$\array{ X \times_Z^\infty Y &\to& Y \\ \downarrow &\swArrow& \downarrow^{\mathrlap{\psi}} \\ X &\underset{\phi}{\to}& Z }$

in an (∞,1)-category of $R$-∞-modules.

## Properties

### Solution spaces of homogeneous $R$-linear equations

We discuss solution space of homogeneous linear equations in the general context of linear algebra over a ring $R$ (not necessarily a field).

So let $R$ be a ring and let $N \in R$Mod be an $R$-module.

Let $n_0,n_1 \in \mathbb{N}$ and let $K = (K_{i j})$ be an an $n_0 \times n_1$ matrix with entries in $R$. By matrix multiplication this defines a linear function

$K \cdot (-) : N^{n_0} \to N^{n_1} \,.$

which takes the element $\vec u = (u_1, \cdots, u_{n_0}) \in N^{n_0}$ to the element $K \cdot \vec u$ with

$(K \cdot \vec u)_i = \sum_{j = 1}^{n_0} K_{i j}\cdot u_j \,.$

Consider dually the linear map

$(-) \cdot K^T : R^{n_1} \to R^{n_0}$

on the free modules over $R$.

Consider the quotient module of $R^{n_1}$ by the image of this map

$R^{n_1}/ (R^{n_0} \cdot K^T) \,,$

hence the cokernel of the map, fitting in the exact sequence

$R^{n_1} \stackrel{(-)\cdot K}{\to} R^{n_0} \to R^{n_1}/(R^{n_1}\cdot K^T) \to 0$

Here the morphism on the left is also called the inclusion of the syzygies of the module on the right.

Applying the left exact hom functor $Hom_{R Mod}(-,N)$ to this yields exact sequence

$0 \to Hom_{R Mod}(R^{n_1}/(R^{n_0}\cdot K^T), N) \to N^{n_0} \stackrel{K \cdot(-)}{\to} N^{n_1} \,.$

This identifies $Hom_{R Mod}(R^{n_1}/(R^{n_0}\cdot K), N)$ as the space of solutions of the homogeneous linear equation $K \cdot \vec u = 0$.

(…)

### Relation to syzygies and projective resolutions of modules

For $R$ a ring, there is close relation between

1. $R$-linear equations in finitely many variables;

2. finitely generated $R$-modules;

3. syzygies in these $R$-modules

4. and projective resolutions of these $R$-modules.

These relations we discuss in the following.

(…)

## References

Discussion in the context of syzygies and projective resolutions of modules is for instance in section 4.5 of

Revised on September 24, 2012 23:07:11 by Tim Porter (95.147.237.35)