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The term rng was coined to alleviate this ambiguity when people want to refer explicitly to a ring without the axiom of multiplicative identity. A number of algebras of functions considered in analysis are not unital, for instance the algebra of functions decreasing to zero at infinity, especially those with compact support on some (non-compact ...
An example of a non-multiplicative function is the arithmetic function r 2 (n) - the number of representations of n as a sum of squares of two integers, positive, negative, or zero, where in counting the number of ways, reversal of order is allowed. For example:
To give an example, let S be the ring of all functions from R to itself; the addition and the multiplication are those of functions. Let x be the identity function. Each r in R defines a constant function, giving rise to the homomorphism R → S. The universal property says that this map extends uniquely to
Thomae's function: is a function that is continuous at all irrational numbers and discontinuous at all rational numbers. It is also a modification of Dirichlet function and sometimes called Riemann function. Kronecker delta function: is a function of two variables, usually integers, which is 1 if they are equal, and 0 otherwise.
The function f : Z → Z/nZ, defined by f(a) = [a] n = a mod n is a surjective ring homomorphism with kernel nZ (see modular arithmetic). The complex conjugation C → C is a ring homomorphism (this is an example of a ring automorphism). For a ring R of prime characteristic p, R → R, x → x p is a ring endomorphism called the Frobenius ...
In number theory, functions of positive integers which respect products are important and are called completely multiplicative functions or totally multiplicative functions. A weaker condition is also important, respecting only products of coprime numbers, and such functions are called multiplicative functions. Outside of number theory, the ...
As another example, φ(1) = 1 since for n = 1 the only integer in the range from 1 to n is 1 itself, and gcd(1, 1) = 1. Euler's totient function is a multiplicative function, meaning that if two numbers m and n are relatively prime, then φ(mn) = φ(m)φ(n).
A ring in which the zero-product property holds is called a domain.A commutative domain with a multiplicative identity element is called an integral domain.Any field is an integral domain; in fact, any subring of a field is an integral domain (as long as it contains 1).