# Exterior products

The *exterior product* is the fundamental product of Grassmann Algebra, and it forms part of the geometric product in geometric algebra. There are two products with symmetric properties called the exterior product and exterior antiproduct.

The exterior product between two elements $$\mathbf a$$ and $$\mathbf b$$ generally combines their spatial extents, and the magnitude of the result indicates how close they are to being orthogonal. If the spatial extents of $$\mathbf a$$ and $$\mathbf b$$ are parallel, then their exterior product is zero. (Compare this to the dot product, which is zero whenever $$\mathbf a$$ and $$\mathbf b$$ are orthogonal.)

Scalars, regarded as multiples of the basis element $${\mathbf 1}$$, are orthogonal to everything other than scalars. The exterior product between two scalars is zero, but the exterior product between a scalar $$a$$ and any other basis element $$\mathbf b$$ is $$a\mathbf b$$, as shown in the table below.

## Exterior Product

The exterior product is widely known as the *wedge product* because it is written with an upward pointing wedge. The exterior product between elements $$\mathbf a$$ and $$\mathbf b$$ is written $$\mathbf a \wedge \mathbf b$$ and read "$$\mathbf a$$ wedge $$\mathbf b$$". Grassmann called this the progressive combinatorial product.

The defining characteristic of the wedge product is that multiplying any vector $$\mathbf v$$ by itself produces zero: $$\mathbf v \wedge \mathbf v = 0$$. This implies that the wedge product is anticommutative for vectors, so we always have

- $$\mathbf v \wedge \mathbf w = -\mathbf w \wedge \mathbf v$$

for vectors $$\mathbf v$$ and $$\mathbf w$$. The wedge product is not anticommutative in general, however. For general basis elements $$\mathbf a$$ and $$\mathbf b$$, reversing the order of the operands satisfies the relationship

- $$\mathbf a \wedge \mathbf b = (-1)^{\operatorname{gr}(\mathbf a)\operatorname{gr}(\mathbf b)} \mathbf b \wedge \mathbf a$$ .

The wedge product adds the grades of its operands, so we have

- $$\operatorname{gr}(\mathbf a \wedge \mathbf b) = \operatorname{gr}(\mathbf a) + \operatorname{gr}(\mathbf b)$$ .

The following Cayley table shows the exterior products between all pairs of basis elements in the 4D projective geometric algebra $$\mathcal G_{3,0,1}$$.

## Exterior Antiproduct

The exterior antiproduct is a dual to the exterior product. It is written with a downward pointing wedge and thus called the *antiwedge product*. The exterior antiproduct $$\mathbf a \vee \mathbf b$$ is read "$$\mathbf a$$ antiwedge $$\mathbf b$$". Grassmann called this the regressive combinatorial product.

Whereas the wedge product combines the full dimensions of its operands, the antiwedge product combines the empty dimensions of its operands. The antiwedge product adds the antigrades of its operands, so we have

- $$\operatorname{ag}(\mathbf a \vee \mathbf b) = \operatorname{ag}(\mathbf a) + \operatorname{ag}(\mathbf b)$$ .

The following Cayley table shows the exterior antiproducts between all pairs of basis elements in the 4D projective geometric algebra $$\mathcal G_{3,0,1}$$.

## De Morgan Laws

There are many possible exterior products and antiproducts. The signs of the results they produce differ in grade-dependent ways, but are otherwise equivalent. The relationship between the product and antiproduct is fixed by a specific choice of dualization function that exchanges full and empty dimensions. We choose the left and right complements as the dualization function and its inverse. We can then express each product in terms of the other through an analog of De Morgan's laws as follows.

- $$\overline{\mathbf a \wedge \mathbf b} = \overline{\mathbf{a\vphantom{b}}} \vee \overline{\mathbf b}$$

- $$\overline{\mathbf a \vee \mathbf b} = \overline{\mathbf{a\vphantom{b}}} \wedge \overline{\mathbf b}$$

- $$\underline{\mathbf a \wedge \mathbf b} = \underline{\mathbf{a\vphantom{b}}} \vee \underline{\mathbf b}$$

- $$\underline{\mathbf a \vee \mathbf b} = \underline{\mathbf{a\vphantom{b}}} \wedge \underline{\mathbf b}$$

## General Properties

The following table lists several general properties of the wedge product and antiwedge product.

Property | Description |
---|---|

$$\mathbf a \wedge \mathbf b = -\mathbf b \wedge \mathbf a$$ | Anticommutativity of the wedge product for vectors $$\mathbf a$$ and $$\mathbf b$$. |

$$\mathbf a \vee \mathbf b = -\mathbf b \vee \mathbf a$$ | Anticommutativity of the antiwedge product for antivectors $$\mathbf a$$ and $$\mathbf b$$. |

$$(\mathbf a \wedge \mathbf b) \wedge \mathbf c = \mathbf a \wedge (\mathbf b \wedge \mathbf c)$$ | Associative law for the wedge product for any non-scalar elements $$\mathbf a$$, $$\mathbf b$$, and $$\mathbf c$$. |

$$(\mathbf a \vee \mathbf b) \vee \mathbf c = \mathbf a \vee (\mathbf b \vee \mathbf c)$$ | Associative law for the antiwedge product for any non-antiscalar elements $$\mathbf a$$, $$\mathbf b$$, and $$\mathbf c$$. |

$$\mathbf a \wedge (\mathbf b + \mathbf c) = \mathbf a \wedge \mathbf b + \mathbf a \wedge \mathbf c$$ | Distributive law for the wedge product for any elements $$\mathbf a$$, $$\mathbf b$$, and $$\mathbf c$$. |

$$\mathbf a \vee (\mathbf b + \mathbf c) = \mathbf a \vee \mathbf b + \mathbf a \vee \mathbf c$$ | Distributive law for the antiwedge product for any elements $$\mathbf a$$, $$\mathbf b$$, and $$\mathbf c$$. |

$$(t\mathbf a) \wedge \mathbf b = \mathbf a \wedge (t\mathbf b) = t(\mathbf a \wedge \mathbf b)$$ | Scalar factorization of the wedge product. |

$$(t\mathbf a) \vee \mathbf b = \mathbf a \vee (t\mathbf b) = t(\mathbf a \vee \mathbf b)$$ | Scalar factorization of the antiwedge product. |

$$s \wedge \mathbf a = \mathbf a \wedge s = s\mathbf a$$ | Wedge product of a scalar $$s$$ and any basis element $$\mathbf a$$ that is not a scalar. |

$$s \vee \mathbf a = \mathbf a \vee s = s\mathbf a$$ | Antiwedge product of an antiscalar $$s$$ and any basis element $$\mathbf a$$ that is not an antiscalar. |

$$s \wedge t = 0$$ | Wedge product of scalars $$s$$ and $$t$$. |

$$s \vee t = 0$$ | Antiwedge product of antiscalars $$s$$ and $$t$$. |