theory of abelian groups

**natural deduction** metalanguage, practical foundations

**type theory** (dependent, intensional, observational type theory, homotopy type theory)

**computational trinitarianism** =

**propositions as types** +**programs as proofs** +**relation type theory/category theory**

The *theory of abelian groups* $T$ is the logical theory whose models in a cartesian category are the abelian group objects.

The *theory of abelian groups* $T$ is the theory over the signature with one sort $X$, one constant $0$, two function symbols $+$ (of sort $X\times X\to X$) and $-$ (of sort $X\to X$) and equality with axioms:

- $\top\vdash_x x+0=x \quad$
- $\top\vdash_{x,y,z} x+(y+z)=(x+y)+z\quad$,
- $\top\vdash_x x+(-x)=0\quad$.
- $\top\vdash_{x,y} x+y=y+x\quad$.

The theory of abelian groups is algebraic. In the following we list some extensions that use increasingly stronger fragments of geometric logic.

The **theory of torsion-free abelian groups** $T^\infty$ results from $T$ by addition of the following axioms:

- For all $n\geq 2$: $((nx=0))\vdash_x (x=0))\quad$.

The resulting theory is a Horn theory.

The **theory of divisible abelian groups** $T^\backslash$ results from $T$ by addition of the following regular axioms:

- For all $n\geq 2$: $\top\vdash_x (\exists y) (ny=x)\quad$.

The **theory of divisible torsion-free abelian groups** $T^{\backslash\infty}$ results from $T^\backslash$ by adding the above axioms for torsion-freeness. The resulting theory is cartesian.

The **theory of torsion abelian groups** $T^~$ is an infinitary geometric theory resulting from $T$ by addition of:

- $\top\vdash_x \bigvee_{n\geq 2} (nx=0)\quad$.

- Peter Johnstone,
*Sketches of an Elephant II*, Oxford UP 2002. pp.812-815.

Last revised on October 29, 2014 at 12:58:28. See the history of this page for a list of all contributions to it.