In mathematics, a product is the result of multiplication, or an expression that identifies objects (numbers or variables) to be multiplied, called factors. For example, 30 is the product of 6 and 5 (the result of multiplication), and is the product of and (indicating that the two factors should be multiplied together). The order in which real or complex numbers are multiplied has no bearing on the product; this is known as the commutative law of multiplication. When matrices or members of various other associative algebras are multiplied, the product usually depends on the order of the factors. Matrix multiplication, for example, is non-commutative, and so is multiplication in other algebras in general as well. There are many different kinds of products in mathematics: besides being able to multiply just numbers, polynomials or matrices, one can also define products on many different algebraic structures. Product of two numbers Product of a sequence The product operator for the product of a sequence is denoted by the capital Greek letter pi Π (in analogy to the use of the capital Sigma Σ as summation symbol). For example, the expression is another way of writing . The product of a sequence consisting of only one number is just that number itself; the product of no factors at all is known as the empty product, and is equal to 1. Commutative rings Commutative rings have a product operation. Residue classes of integers Residue classes in the rings can be added: and multiplied: Convolution Two functions from the reals to itself can be multiplied in another way, called the convolution. If then the integral is well defined and is called the convolution. Under the Fourier transform, convolution becomes point-wise function multiplication. Polynomial rings The product of two polynomials is given by the following: with Products in linear algebra There are many different kinds of products in linear algebra. Some of these have confusingly similar names (outer product, exterior product) with very different meanings, while others have very different names (outer product, tensor product, Kronecker product) and yet convey essentially the same idea. A brief overview of these is given in the following sections. Scalar multiplication By the very definition of a vector space, one can form the product of any scalar with any vector, giving a map . Scalar product A scalar product is a bi-linear map: with the following conditions, that for all .