The additive neutral element in the natural numbers, integers, real numbers and complex numbers is called zero and written as 00.

More generally, in any abelian group, or even any commutative monoid, the group operation is often called ‘addition’ and written as ++, and then the neutral element is called zero and written as 00. As a consequence, in any ring, or more generally any rig, the two binary operations are called ‘multiplication’ and ‘addition’, and the identity for addition is called zero.


Categorifying this idea, in any 2-rig the additive identity may be called zero. This is especially true in the case of a distributive category, that is a category with (at least finitary) products and coproducts, the former distributing over the latter. In this case the initial object, which serves as the identity for coproducts, is often called zero:

x+0xx + 0 \cong x

For example, in the category Set, the empty set is often written 00 in the category-theoretic literature.

In an abelian category, the initial object is also terminal, and denoted 00. More generally, any object with this property is called a zero object.

Categorifying horizontally instead, we get the notion of zero morphism.

All these ideas can be, and have been, categorified further.

Last revised on April 19, 2018 at 02:35:00. See the history of this page for a list of all contributions to it.