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In algebra, an additive map, Z-linear map or additive function is a function f that preserves the addition operation: f(x + y) = f(x) + f(y) for every pair of elements x and y in the domain of f. For example, any linear map is additive. When the domain is the
real number In mathematics, a real number is a number that can be used to measure a continuous one- dimensional quantity such as a duration or temperature. Here, ''continuous'' means that pairs of values can have arbitrarily small differences. Every re ...
s, this is Cauchy's functional equation. For a specific case of this definition, see additive polynomial. More formally, an additive map is a \Z- module homomorphism. Since an abelian group is a \Z- module, it may be defined as a group homomorphism between abelian groups. A map V \times W \to X that is additive in each of two arguments separately is called a bi-additive map or a \Z-bilinear map.


Examples

Typical examples include maps between rings,
vector space In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called vector (mathematics and physics), ''vectors'', can be added together and multiplied ("scaled") by numbers called sc ...
s, or modules that preserve the additive group. An additive map does not necessarily preserve any other structure of the object; for example, the product operation of a ring. If f and g are additive maps, then the map f + g (defined pointwise) is additive.


Properties

Definition of scalar multiplication by an integer Suppose that X is an additive group with identity element 0 and that the inverse of x \in X is denoted by -x. For any x \in X and integer n \in \Z, let: n x := \left\{ \begin{alignat}{9} & &&0 && && &&~~~~ && &&~\text{ when } n = 0, \\ & &&x &&+ \cdots + &&x &&~~~~ \text{(} n &&\text{ summands) } &&~\text{ when } n > 0, \\ & (-&&x) &&+ \cdots + (-&&x) &&~~~~ \text{(} , n, &&\text{ summands) } &&~\text{ when } n < 0, \\ \end{alignat} \right. Thus (-1) x = - x and it can be shown that for all integers m, n \in \Z and all x \in X, (m + n) x = m x + n x and - (n x) = (-n) x = n (-x). This definition of scalar multiplication makes the cyclic subgroup \Z x of X into a left \Z-module; if X is commutative, then it also makes X into a left \Z-module. Homogeneity over the integers If f : X \to Y is an additive map between additive groups then f(0) = 0 and for all x \in X, f(-x) = - f(x) (where negation denotes the additive inverse) andf(0) = f(0 + 0) = f(0) + f(0) so adding -f(0) to both sides proves that f(0) = 0. If x \in X then 0 = f(0) = f(x + (-x)) = f(x) + f(-x) so that f(-x) = - f(x) where by definition, (-1) f(x) := - f(x). Induction shows that if n \in \N is positive then f(n x) = n f(x) and that the additive inverse of n f(x) is n (- f(x)), which implies that f((-n) x) = f(n (-x)) = n f(-x) = n (- f(x)) = -(n f(x)) = (-n) f(x) (this shows that f(n x) = n f(x) holds for n < 0). \blacksquare f(n x) = n f(x) \quad \text{ for all } n \in \Z. Consequently, f(x - y) = f(x) - f(y) for all x, y \in X (where by definition, x - y := x + (-y)). In other words, every additive map is homogeneous over the integers. Consequently, every additive map between abelian groups is a homomorphism of \Z-modules. Homomorphism of \Q-modules If the additive abelian groups X and Y are also a unital modules over the rationals \Q (such as real or complex
vector space In mathematics and physics, a vector space (also called a linear space) is a set (mathematics), set whose elements, often called vector (mathematics and physics), ''vectors'', can be added together and multiplied ("scaled") by numbers called sc ...
s) then an additive map f : X \to Y satisfies:Let x \in X and q = \frac{m}{n} \in \Q where m, n \in \Z and n > 0. Let y := \frac{1}{n} x. Then n y = n \left(\frac{1}{n} x\right) = \left(n \frac{1}{n}\right) x = (1) x = x, which implies f(x) = f(n y) = n f(y) = n f\left(\frac{1}{n} x\right) so that multiplying both sides by \frac{1}{n} proves that f\left(\frac{1}{n} x\right) = \frac{1}{n} f(x). Consequently, f(q x) = f\left(\frac{m}{n} x\right) = m f\left(\frac{1}{n} x\right) = m\left(\frac{1}{n} f(x)\right) = q f(x). \blacksquare f(q x) = q f(x) \quad \text{ for all } q \in \Q \text{ and } x \in X. In other words, every additive map is homogeneous over the rational numbers. Consequently, every additive maps between unital \Q-modules is a homomorphism of \Q-modules. Despite being homogeneous over \Q, as described in the article on Cauchy's functional equation, even when X = Y = \R, it is nevertheless still possible for the additive function f : \R \to \R to be homogeneous over the real numbers; said differently, there exist additive maps f : \R \to \R that are of the form f(x) = s_0 x for some constant s_0 \in \R. In particular, there exist additive maps that are not linear maps.


See also

*


Notes

Proofs


References

* {{citation, author1= Roger C. Lyndon, author2= Paul E. Schupp, title=Combinatorial Group Theory, publisher=Springer, year=2001 Ring theory Morphisms Types of functions