In linear algebra, the **quotient** of a vector space by a subspace is a vector space obtained by "collapsing" to zero. The space obtained is called a **quotient space** and is denoted (read " mod " or " by ").

Formally, the construction is as follows.^{[1]} Let be a vector space over a field , and let be a subspace of . We define an equivalence relation on by stating that if . That is, is related to if one can be obtained from the other by adding an element of . From this definition, one can deduce that any element of is related to the zero vector; more precisely, all the vectors in get mapped into the equivalence class of the zero vector.

The equivalence class – or, in this case, the coset – of is often denoted

since it is given by

The quotient space is then defined as , the set of all equivalence classes induced by on . Scalar multiplication and addition are defined on the equivalence classes by^{[2]}^{[3]}

- for all , and
- .

It is not hard to check that these operations are well-defined (i.e. do not depend on the choice of representatives). These operations turn the quotient space into a vector space over with being the zero class, .

The mapping that associates to the equivalence class is known as the **quotient map**.

Alternatively phrased, the quotient space is the set of all affine subsets of which are parallel to .^{[4]}

Let *X* = **R**^{2} be the standard Cartesian plane, and let *Y* be a line through the origin in *X*. Then the quotient space *X*/*Y* can be identified with the space of all lines in *X* which are parallel to *Y*. That is to say that, the elements of the set *X*/*Y* are lines in *X* parallel to *Y*. Note that the points along any one such line will satisfy the equivalence relation because their difference vectors belong to *Y*. This gives a way to visualize quotient spaces geometrically. (By re-parameterising these lines, the quotient space can more conventionally be represented as the space of all points along a line through the origin that is not parallel to *Y*. Similarly, the quotient space for **R**^{3} by a line through the origin can again be represented as the set of all co-parallel lines, or alternatively be represented as the vector space consisting of a plane which only intersects the line at the origin.)

Another example is the quotient of **R**^{n} by the subspace spanned by the first *m* standard basis vectors. The space **R**^{n} consists of all *n*-tuples of real numbers (*x*_{1}, ..., *x*_{n}). The subspace, identified with **R**^{m}, consists of all *n*-tuples such that the last *n* − *m* entries are zero: (*x*_{1}, ..., *x*_{m}, 0, 0, ..., 0). Two vectors of **R**^{n} are in the same equivalence class modulo the subspace if and only if they are identical in the last *n* − *m* coordinates. The quotient space **R**^{n}/**R**^{m} is isomorphic to **R**^{n−m} in an obvious manner.

Let be the vector space of all cubic polynomials over the real numbers. Then is a quotient space, where each element is the set corresponding to polynomials that differ by a quadratic term only. For example, one element of the quotient space is , while another element of the quotient space is .

More generally, if *V* is an (internal) direct sum of subspaces *U* and *W,*

then the quotient space *V*/*U* is naturally isomorphic to *W*.^{[5]}

An important example of a functional quotient space is an L^{p} space.

There is a natural epimorphism from *V* to the quotient space *V*/*U* given by sending *x* to its equivalence class [*x*]. The kernel (or nullspace) of this epimorphism is the subspace *U*. This relationship is neatly summarized by the short exact sequence

If *U* is a subspace of *V*, the dimension of *V*/*U* is called the **codimension** of *U* in *V*. Since a basis of *V* may be constructed from a basis *A* of *U* and a basis *B* of *V*/*U* by adding a representative of each element of *B* to *A*, the dimension of *V* is the sum of the dimensions of *U* and *V*/*U*. If *V* is finite-dimensional, it follows that the codimension of *U* in *V* is the difference between the dimensions of *V* and *U*:^{[6]}^{[7]}

Let *T* : *V* → *W* be a linear operator. The kernel of *T*, denoted ker(*T*), is the set of all *x* in *V* such that *Tx* = 0. The kernel is a subspace of *V*. The first isomorphism theorem for vector spaces says that the quotient space *V*/ker(*T*) is isomorphic to the image of *V* in *W*. An immediate corollary, for finite-dimensional spaces, is the rank–nullity theorem: the dimension of *V* is equal to the dimension of the kernel (the nullity of *T*) plus the dimension of the image (the rank of *T*).

The cokernel of a linear operator *T* : *V* → *W* is defined to be the quotient space *W*/im(*T*).

If *X* is a Banach space and *M* is a closed subspace of *X*, then the quotient *X*/*M* is again a Banach space. The quotient space is already endowed with a vector space structure by the construction of the previous section. We define a norm on *X*/*M* by

Let *C*[0,1] denote the Banach space of continuous real-valued functions on the interval [0,1] with the sup norm. Denote the subspace of all functions *f* ∈ *C*[0,1] with *f*(0) = 0 by *M*. Then the equivalence class of some function *g* is determined by its value at 0, and the quotient space *C*[0,1]/*M* is isomorphic to **R**.

If *X* is a Hilbert space, then the quotient space *X*/*M* is isomorphic to the orthogonal complement of *M*.

The quotient of a locally convex space by a closed subspace is again locally convex.^{[8]} Indeed, suppose that *X* is locally convex so that the topology on *X* is generated by a family of seminorms {*p*_{α} | α ∈ *A*} where *A* is an index set. Let *M* be a closed subspace, and define seminorms *q*_{α} on *X*/*M* by

Then *X*/*M* is a locally convex space, and the topology on it is the quotient topology.

If, furthermore, *X* is metrizable, then so is *X*/*M*. If *X* is a Fréchet space, then so is *X*/*M*.^{[9]}