# nLab near-ring

Near-rings

### Context

#### Algebra

higher algebra

universal algebra

# Near-rings

## Idea

A near-ring is a sort of ring in which addition need not be commutative. However, if we simply remove the commutativity of addition from the usual definition of a ring, then nothing changes, because we can prove commutativity from the other axioms: we have

$(x+y)(1+1) = x(1+1) + y(1+1) = x + x + y + y$

and also

$(x+y)(1+1) = (x+y)1 + (x+y)1 = x + y + x + y.$

Canceling $x$ on the left and $y$ on the right, we have $x+y=y+x$.

Thus, in order for the notion of near-ring to be different from that of a ring, we need to relax the distributivity law as well; we impose it only on one side.

## Definition

A near-ring is a set $R$ equipped with

1. A group structure $(R,+,0)$,

2. A monoid structure $(R,\cdot,1)$,

3. such that for any $x,y,z\in R$ we have $(x+y)\cdot z = (x\cdot z) + (y\cdot z)$, and for any $x\in R$ we have $0\cdot x = 0$.

If $(R,+,0)$ is only a monoid or semigroup, we say instead that $R$ is a near-rig or a near-semiring. (Of course, now it is possible to have distributivity on both sides without making addition commutative, since addition need not always cancel.)

There is also a weaker notion, of loop near ring, where in the axioms above the group structure on $(R,+,0)$ is weakened to a loop algebraic structure (i.e. the associativity of addition is dropped).

## Internalization

Of course, near-rings can be defined internally to any cartesian monoidal category. More generally, they can be defined internally to a braided duoidal category.

For loop near rings see

• D. Ramakotaiah, C. Santhakumari. On loop near-rings. Bull. Austral. Math. Soc. 19(3):417{435, 1978.

Loop near rings appear in topology:

• Damir Franetič, Petar Pavešić, Loop near-rings and unique decompositions of H-spaces, arxiv/1511.06168
category: algebra