Cancellation property

In mathematics, the notion of cancellativity (or ''cancellability'') is a generalization of the notion of invertibility.

An element ''a'' in a magma has the left cancellation property (or is left-cancellative) if for all ''b'' and ''c'' in ''M'', always implies that .

An element ''a'' in a magma has the right cancellation property (or is right-cancellative) if for all ''b'' and ''c'' in ''M'', always implies that .

An element ''a'' in a magma has the two-sided cancellation property (or is cancellative) if it is both left- and right-cancellative.

A magma is left-cancellative if all ''a'' in the magma are left cancellative, and similar definitions apply for the right cancellative or two-sided cancellative properties.

In a semigroup, a left-invertible element is left-cancellative, and analogously for right and two-sided. If ''a''⁻¹ is the left inverse of ''a'', then implies , which implies by associativity.

For example, every quasigroup, and thus every group, is cancellative.

Interpretation

To say that an element $a$ in a magma is left-cancellative, is to say that the function is injective where is also an element of . That the function ''g'' is injective implies that given some equality of the form , where the only unknown is ''x'', there is only one possible value of ''x'' satisfying the equality. More precisely, we are able to define some function ''f'', the inverse of ''g'', such that for all ''x,'' . Put another way, for all ''x'' and ''y'' in ''M'', if , then .

Similarly, to say that the element ''a'' is right-cancellative, is to say that the function is injective and that for all ''x'' and ''y'' in ''M'', if , then .

Examples of cancellative monoids and semigroups

The positive (equally non-negative) integers form a cancellative semigroup under addition. The non-negative integers form a cancellative monoid under addition. Each of these is an example of a cancellative magma that is not a quasigroup.

Any free semigroup or monoid obeys the cancellative law, and in general, any semigroup or monoid that embeds into a group (as the above examples clearly do) will obey the cancellative law.

In a different vein, (a subsemigroup of) the multiplicative semigroup of elements of a ring that are not zero divisors (which is just the set of all nonzero elements if the ring in question is a domain, like the integers) has the cancellation property. This remains valid even if the ring in question is noncommutative and/or nonunital.

Non-cancellative algebraic structures

Although the cancellation law holds for addition, subtraction, multiplication and division of real and complex numbers (with the single exception of multiplication by zero and division of zero by another number), there are a number of algebraic structures where the cancellation law is not valid.

The cross product of two vectors does not obey the cancellation law. If , then it does not follow that even if (take for example)

Matrix multiplication also does not necessarily obey the cancellation law. If and , then one must show that matrix A is ''invertible'' (i.e. has ) before one can conclude that . If , then B might not equal C, because the matrix equation will not have a unique solution for a non-invertible matrix A.

Also note that if and and the matrix A is ''invertible'' (i.e. has ), it is not necessarily true that . Cancellation works only for and (provided that matrix A is ''invertible'') and not for and .

See also

* Grothendieck group
* Invertible element
* Cancellative semigroup
* Integral domain

References

{{DEFAULTSORT:Cancellation Property
Non-associative algebra
Properties of binary operations
Algebraic properties of elements

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