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In mathematics, a **quaternion algebra** over a field *F* is a central simple algebra *A* over *F*^{[1]}^{[2]} that has dimension 4 over *F*. Every quaternion algebra becomes a matrix algebra by *extending scalars* (equivalently, tensoring with a field extension), i.e. for a suitable field extension *K* of *F*, is isomorphic to the 2 × 2 matrix algebra over *K*.

The notion of a quaternion algebra can be seen as a generalization of Hamilton's quaternions to an arbitrary base field. The Hamilton quaternions are a quaternion algebra (in the above sense) over , and indeed the only one over apart from the 2 × 2 real matrix algebra, up to isomorphism. When , then the biquaternions form the quaternion algebra over *F*.

*Quaternion algebra* here means something more general than the algebra of Hamilton's quaternions. When the coefficient field *F* does not have characteristic 2, every quaternion algebra over *F* can be described as a 4-dimensional *F*-vector space with basis , with the following multiplication rules:

where *a* and *b* are any given nonzero elements of *F*. From these rules we get:

The classical instances where are Hamilton's quaternions (*a* = *b* = −1) and split-quaternions (*a* = −1, *b* = +1). In split-quaternions, and , differing from Hamilton's equations.

The algebra defined in this way is denoted (*a*,*b*)_{F} or simply (*a*,*b*).^{[3]} When *F* has characteristic 2, a different explicit description in terms of a basis of 4 elements is also possible, but in any event the definition of a quaternion algebra over *F* as a 4-dimensional central simple algebra over *F* applies uniformly in all characteristics.

A quaternion algebra (*a*,*b*)_{F} is either a division algebra or isomorphic to the matrix algebra of 2 × 2 matrices over *F*; the latter case is termed *split*.^{[4]} The *norm form*

defines a structure of division algebra if and only if the norm is an anisotropic quadratic form, that is, zero only on the zero element. The conic *C*(*a*,*b*) defined by

has a point (*x*,*y*,*z*) with coordinates in *F* in the split case.^{[5]}

Quaternion algebras are applied in number theory, particularly to quadratic forms. They are concrete structures that generate the elements of order two in the Brauer group of *F*. For some fields, including algebraic number fields, every element of order 2 in its Brauer group is represented by a quaternion algebra. A theorem of Alexander Merkurjev implies that each element of order 2 in the Brauer group of any field is represented by a tensor product of quaternion algebras.^{[6]} In particular, over *p*-adic fields the construction of quaternion algebras can be viewed as the quadratic Hilbert symbol of local class field theory.

It is a theorem of Frobenius that there are only two real quaternion algebras: 2 × 2 matrices over the reals and Hamilton's real quaternions.

In a similar way, over any local field *F* there are exactly two quaternion algebras: the 2 × 2 matrices over *F* and a division algebra.
But the quaternion division algebra over a local field is usually *not* Hamilton's quaternions over the field. For example, over the *p*-adic numbers Hamilton's quaternions are a division algebra only when *p* is 2. For odd prime *p*, the *p*-adic Hamilton quaternions are isomorphic to the 2 × 2 matrices over the *p*-adics. To see the *p*-adic Hamilton quaternions are not a division algebra for odd prime *p*, observe that the congruence *x*^{2} + *y*^{2} = −1 mod *p* is solvable and therefore by Hensel's lemma — here is where *p* being odd is needed — the equation

*x*^{2}+*y*^{2}= −1

is solvable in the *p*-adic numbers. Therefore the quaternion

*xi*+*yj*+*k*

has norm 0 and hence doesn't have a multiplicative inverse.

One way to classify the *F*-algebra isomorphism classes of all quaternion algebras for a given field *F* is to use the one-to-one correspondence between isomorphism classes of quaternion algebras over *F* and isomorphism classes of their *norm forms*.

To every quaternion algebra *A*, one can associate a quadratic form *N* (called the *norm form*) on *A* such that

for all *x* and *y* in *A*. It turns out that the possible norm forms for quaternion *F*-algebras are exactly the Pfister 2-forms.

Quaternion algebras over the rational numbers have an arithmetic theory similar to, but more complicated than, that of quadratic extensions of .

Let be a quaternion algebra over and let be a place of , with completion (so it is either the *p*-adic numbers for some prime *p* or the real numbers ). Define , which is a quaternion algebra over . So there are two choices for
: the 2 × 2 matrices over or a division algebra.

We say that is **split** (or **unramified**) at if is isomorphic to the 2 × 2 matrices over . We say that *B* is **non-split** (or **ramified**) at if is the quaternion division algebra over . For example, the rational Hamilton quaternions is non-split at 2 and at and split at all odd primes. The rational 2 × 2 matrices are split at all places.

A quaternion algebra over the rationals which splits at is analogous to a real quadratic field and one which is non-split at is analogous to an imaginary quadratic field. The analogy comes from a quadratic field having real embeddings when the minimal polynomial for a generator splits over the reals and having non-real embeddings otherwise. One illustration of the strength of this analogy concerns unit groups in an order of a rational quaternion algebra:
it is infinite if the quaternion algebra splits at ^{[citation needed]} and it is finite otherwise^{[citation needed]}, just as the unit group of an order in a quadratic ring is infinite in the real quadratic case and finite otherwise.

The number of places where a quaternion algebra over the rationals ramifies is always even, and this is equivalent to the quadratic reciprocity law over the rationals.
Moreover, the places where *B* ramifies determines *B* up to isomorphism as an algebra. (In other words, non-isomorphic quaternion algebras over the rationals do not share the same set of ramified places.) The product of the primes at which *B* ramifies is called the **discriminant** of *B*.

- Gille, Philippe; Szamuely, Tamás (2006).
*Central simple algebras and Galois cohomology*. Cambridge Studies in Advanced Mathematics. Vol. 101. Cambridge: Cambridge University Press. doi:10.1017/CBO9780511607219. ISBN 0-521-86103-9. Zbl 1137.12001. - Lam, Tsit-Yuen (2005).
*Introduction to Quadratic Forms over Fields*. Graduate Studies in Mathematics. Vol. 67. American Mathematical Society. ISBN 0-8218-1095-2. MR 2104929. Zbl 1068.11023.

The Wikibook *Associative Composition Algebra* has a page on the topic of: **Quaternion algebras over R and C**

- Knus, Max-Albert; Merkurjev, Alexander; Rost, Markus; Tignol, Jean-Pierre (1998).
*The book of involutions*. Colloquium Publications. Vol. 44. With a preface by J. Tits. Providence, RI: American Mathematical Society. ISBN 0-8218-0904-0. MR 1632779. Zbl 0955.16001. - Maclachlan, Colin; Reid, Alan W. (2003).
*The Arithmetic of Hyperbolic 3-Manifolds*. New York: Springer-Verlag. doi:10.1007/978-1-4757-6720-9. ISBN 0-387-98386-4. MR 1937957. See chapter 2 (Quaternion Algebras I) and chapter 7 (Quaternion Algebras II). - Chisholm, Hugh, ed. (1911). .
*Encyclopædia Britannica*(11th ed.). Cambridge University Press. (*See section on quaternions.*) *Quaternion algebra*at Encyclopedia of Mathematics.