For matrix multiplication, the number of columns in the first matrix must be equal to the number of rows in the second matrix. The result matrix has the number of rows of the first and the number of columns of the second matrix.
In mathematics, particularly in linear algebra, matrix multiplication is a binary operation that produces a matrix from two matrices. For matrix multiplication, the number of columns in the first matrix must be equal to the number of rows in the second matrix. The resulting matrix, known as the matrix product, has the number of rows of the first and the number of columns of the second matrix. The product of matrices A and B is denoted as AB.
This article will use the following notational conventions: matrices are represented by capital letters in bold, e.g. A; vectors in lowercase bold, e.g. a; and entries of vectors and matrices are italic (they are numbers from a field), e.g. A and a. Index notation is often the clearest way to express definitions, and is used as standard in the literature. The entry in row i, column j of matrix A is indicated by (A)ij, Aij or aij. In contrast, a single subscript, e.g. A1, A2, is used to select a matrix (not a matrix entry) from a collection of matrices.
If A is an m × n matrix and B is an n × p matrix,
the matrix productC = AB (denoted without multiplication signs or dots) is defined to be the m × p matrix
for i = 1, ..., m and j = 1, ..., p.
That is, the entry of the product is obtained by multiplying term-by-term the entries of the ith row of A and the jth column of B, and summing these n products. In other words, is the dot product of the ith row of A and the jth column of B.
Therefore, AB can also be written as
Thus the product AB is defined if and only if the number of columns in A equals the number of rows in B, in this case n.
In most scenarios, the entries are numbers, but they may be any kind of mathematical objects for which an addition and a multiplication are defined, that are associative, and such that the addition is commutative, and the multiplication is distributive with respect to the addition. In particular, the entries may be matrices themselves (see block matrix).
The figure to the right illustrates diagrammatically the product of two matrices A and B, showing how each intersection in the product matrix corresponds to a row of A and a column of B.
The values at the intersections marked with circles are:
Historically, matrix multiplication has been introduced for facilitating and clarifying computations in linear algebra. This strong relationship between matrix multiplication and linear algebra remains fundamental in all mathematics, as well as in physics, chemistry, engineering and computer science.
If a vector space has a finite basis, its vectors are each uniquely represented by a finite sequence of scalars, called a coordinate vector, whose elements are the coordinates of the vector on the basis. These coordinate vectors form another vector space, which is isomorphic to the original vector space. A coordinate vector is commonly organized as a column matrix (also called column vector), which is a matrix with only one column. So, a column vector represents both a coordinate vector, and a vector of the original vector space.
A linear mapA from a vector space of dimension n into a vector space of dimension m maps a column vector
onto the column vector
The linear map A is thus defined by the matrix
and maps the column vector to the matrix product
If B is another linear map from the preceding vector space of dimension m, into a vector space of dimension p, it is represented by a matrix A straightforward computation shows that the matrix of the composite map is the matrix product The general formula ) that defines the function composition is instanced here as a specific case of associativity of matrix product (see § Associativity below):
where the source point and its image are written as column vectors.
The composition of the rotation by and that by then corresponds to the matrix product
where appropriate trigonometric identities are employed for the second equality.
That is, the composition corresponds to the rotation by angle , as expected.
Resource allocation in economics
The computation of the bottom left entry of corresponds to the consideration of all paths (highlighted) from basic commodity to final product in the production flow graph.
As an example, a fictitious factory uses 4 kinds of basic commodities [de], to produce 3 kinds of intermediate goods, , which in turn are used to produce 3 kinds of final products, . The matrices
provide the amount of basic commodities needed for a given amount of intermediate goods, and the amount of intermediate goods needed for a given amount of final products, respectively.
For example, to produce one unit of intermediate good , one unit of basic commodity , two units of , no units of , and one unit of are needed, corresponding to the first column of .
Using matrix multiplication, compute
this matrix directly provides the amounts of basic commodities needed for given amounts of final goods. For example, the bottom left entry of is computed as , reflecting that units of are needed to produce one unit of . Indeed, one unit is needed for , 2 for , and for each of the two units that go into the unit, see picture.
In order to produce e.g. 100 units of the final product , 80 units of , and 60 units of , the necessary amounts of basic goods can be computed as
that is, units of , units of , units of , units of are needed.
Similarly, the product matrix can be used to compute the needed amounts of basic goods for other final-good amount data.
where denotes the conjugate transpose of (conjugate of the transpose, or equivalently transpose of the conjugate).
Matrix multiplication shares some properties with usual multiplication. However, matrix multiplication is not defined if the number of columns of the first factor differs from the number of rows of the second factor, and it is non-commutative, even when the product remains definite after changing the order of the factors.
An operation is commutative if, given two elements A and B such that the product is defined, then is also defined, and
If A and B are matrices of respective sizes and , then is defined if , and is defined if . Therefore, if one of the products is defined, the other one need not be defined. If , the two products are defined, but have different sizes; thus they cannot be equal. Only if , that is, if A and B are square matrices of the same size, are both products defined and of the same size. Even in this case, one has in general
This example may be expanded for showing that, if A is a matrix with entries in a fieldF, then for every matrix B with entries in F, if and only if where , and I is the identity matrix. If, instead of a field, the entries are supposed to belong to a ring, then one must add the condition that c belongs to the center of the ring.
One special case where commutativity does occur is when D and E are two (square) diagonal matrices (of the same size); then DE = ED. Again, if the matrices are over a general ring rather than a field, the corresponding entries in each must also commute with each other for this to hold.
The matrix product is distributive with respect to matrix addition. That is, if A, B, C, D are matrices of respective sizes m × n, n × p, n × p, and p × q, one has (left distributivity)
This results from the distributivity for coefficients by
Product with a scalar
If A is a matrix and c a scalar, then the matrices and are obtained by left or right multiplying all entries of A by c. If the scalars have the commutative property, then
If the product is defined (that is, the number of columns of A equals the number of rows of B), then
If the scalars have the commutative property, then all four matrices are equal. More generally, all four are equal if c belongs to the center of a ring containing the entries of the matrices, because in this case, cX = Xc for all matrices X.
These properties result from the bilinearity of the product of scalars:
If the scalars have the commutative property, the transpose of a product of matrices is the product, in the reverse order, of the transposes of the factors. That is
where T denotes the transpose, that is the interchange of rows and columns.
This identity does not hold for noncommutative entries, since the order between the entries of A and B is reversed, when one expands the definition of the matrix product.
This results from applying to the definition of matrix product the fact that the conjugate of a sum is the sum of the conjugates of the summands and the conjugate of a product is the product of the conjugates of the factors.
Transposition acts on the indices of the entries, while conjugation acts independently on the entries themselves. It results that, if A and B have complex entries, one has
where † denotes the conjugate transpose (conjugate of the transpose, or equivalently transpose of the conjugate).
Given three matrices A, B and C, the products (AB)C and A(BC) are defined if and only if the number of columns of A equals the number of rows of B, and the number of columns of B equals the number of rows of C (in particular, if one of the products is defined, then the other is also defined). In this case, one has the associative property
As for any associative operation, this allows omitting parentheses, and writing the above products as
This extends naturally to the product of any number of matrices provided that the dimensions match. That is, if A1, A2, ..., An are matrices such that the number of columns of Ai equals the number of rows of Ai + 1 for i = 1, ..., n – 1, then the product
These properties may be proved by straightforward but complicated summation manipulations. This result also follows from the fact that matrices represent linear maps. Therefore, the associative property of matrices is simply a specific case of the associative property of function composition.
Computational complexity depends on parenthezation
Although the result of a sequence of matrix products does not depend on the order of operation (provided that the order of the matrices is not changed), the computational complexity may depend dramatically on this order.
For example, if A, B and C are matrices of respective sizes 10×30, 30×5, 5×60, computing (AB)C needs 10×30×5 + 10×5×60 = 4,500 multiplications, while computing A(BC) needs 30×5×60 + 10×30×60 = 27,000 multiplications.
Algorithms have been designed for choosing the best order of products, see Matrix chain multiplication. When the number n of matrices increases, it has been shown that the choice of the best order has a complexity of
If n > 1, many matrices do not have a multiplicative inverse. For example, a matrix such that all entries of a row (or a column) are 0 does not have an inverse. If it exists, the inverse of a matrix A is denoted A−1, and, thus verifies
One may raise a square matrix to any nonnegative integer power multiplying it by itself repeatedly in the same way as for ordinary numbers. That is,
Computing the kth power of a matrix needs k – 1 times the time of a single matrix multiplication, if it is done with the trivial algorithm (repeated multiplication). As this may be very time consuming, one generally prefers using exponentiation by squaring, which requires less than 2 log2k matrix multiplications, and is therefore much more efficient.
An easy case for exponentiation is that of a diagonal matrix. Since the product of diagonal matrices amounts to simply multiplying corresponding diagonal elements together, the kth power of a diagonal matrix is obtained by raising the entries to the power k:
The definition of matrix product requires that the entries belong to a semiring, and does not require multiplication of elements of the semiring to be commutative. In many applications, the matrix elements belong to a field, although the tropical semiring is also a common choice for graph shortest path problems. Even in the case of matrices over fields, the product is not commutative in general, although it is associative and is distributive over matrix addition. The identity matrices (which are the square matrices whose entries are zero outside of the main diagonal and 1 on the main diagonal) are identity elements of the matrix product. It follows that the n × n matrices over a ring form a ring, which is noncommutative except if n = 1 and the ground ring is commutative.
Rather surprisingly, this complexity is not optimal, as shown in 1969 by Volker Strassen, who provided an algorithm, now called Strassen's algorithm, with a complexity of 
Strassen's algorithm can be parallelized to further improve the performance.
As of December 2020[update], the best matrix multiplication algorithm is by Josh Alman and Virginia Vassilevska Williams and has complexity O(n2.3728596).
It is not known whether matrix multiplication can be performed in n2 + o(1) time. This would be optimal, since one must read the elements of a matrix in order to multiply it with another matrix.
Since matrix multiplication forms the basis for many algorithms, and many operations on matrices even have the same complexity as matrix multiplication (up to a multiplicative constant), the computational complexity of matrix multiplication appears throughout numerical linear algebra and theoretical computer science.
Henry Cohn, Robert Kleinberg, Balázs Szegedy, and Chris Umans. Group-theoretic Algorithms for Matrix Multiplication. arXiv:math.GR/0511460. Proceedings of the 46th Annual Symposium on Foundations of Computer Science, 23–25 October 2005, Pittsburgh, PA, IEEE Computer Society, pp. 379–388.
Henry Cohn, Chris Umans. A Group-theoretic Approach to Fast Matrix Multiplication. arXiv:math.GR/0307321. Proceedings of the 44th Annual IEEE Symposium on Foundations of Computer Science, 11–14 October 2003, Cambridge, MA, IEEE Computer Society, pp. 438–449.