“Composition algebra”的意思、由来-开放百科全书

In mathematics, a composition algebra {{mvar|A}} over a field {{mvar|K}} is a not necessarily associative algebra over {{mvar|K}} together with a nondegenerate quadratic form {{mvar|N}} that satisfies

A composition algebra includes an involution called a conjugation: {{math|x ↦ x^∗}}. The quadratic form {{math|1=N(x) = xx^∗}}, and is often called the norm of the algebra.

A composition algebra {{math|(A, ^∗, N)}} is either a division algebra or a split algebra, depending on the existence of a non-zero {{math|v}} in {{math|A}} such that {{math|1=N(v) = 0}}, called a null vector.^[1] When there is no non-zero null vector, the multiplicative inverse of {{math|x}} is {{math|x^∗/N(x)}}, so the algebra is a division algebra. When there is a non-zero null vector, {{math|N}} is an isotropic quadratic form, and "the algebra splits".

Structure theorem

Every unital composition algebra over a field {{mvar|K}} can be obtained by repeated application of the Cayley–Dickson construction starting from {{mvar|K}} (if the characteristic of {{mvar|K}} is different from {{math|2}}) or a 2-dimensional composition subalgebra (if {{math|1=char(K) = 2}}). The possible dimensions of a composition algebra are {{math|1}}, {{math|2}}, {{math|4}}, and {{math|8}}.^[2]^[2]^[3]

For consistent terminology, algebras of dimension 1 have been called unarion, and those of dimension 2 binarion.^[5]

Instances and usage

When the field {{mvar|K}} is taken to be complex numbers {{math|C}} and the quadratic form {{math|z²}}, then four composition algebras over {{math|C}} are {{math|C itself}}, the bicomplex numbers, the biquaternions (isomorphic to the {{gaps|2|×|2}} complex matrix ring {{math|M(2, C)}}), and the bioctonions {{math|C ⊗ O}}, which are also called complex octonions.

Matrix ring {{math|M(2, C)}} has long been an object of interest, first as biquaternions by

Hamilton (1853), later in the isomorphic matrix form, and especially as Pauli algebra.

The squaring function {{math|1=N(x) = x²}} on the real number field forms the primordial composition algebra.

When the field {{mvar|K}} is taken to be real numbers {{math|R}}, then there are just six other real composition algebras.^[2]{{rp|166}}

In two, four, and eight dimensions there are both a division algebra and a "split algebra":

History

The composition of sums of squares was noted by several early authors. Diophantus was aware of the identity involving the sum of two squares, now called the Brahmagupta–Fibonacci identity, which is also articulated as a property of Euclidean norms of complex numbers when multiplied. Leonhard Euler discussed the four-square identity in 1748, and it led W. R. Hamilton to construct his four-dimensional algebra of quaternions.^[4]{{rp|62}} In 1848 tessarines were described giving first light to bicomplex numbers.

About 1818 Danish scholar Ferdinand Degen displayed the Degen's eight-square identity, which was later connected with norms of elements of the octonion algebra:

In 1919 Leonard Dickson advanced the study of the Hurwitz problem with a survey of efforts to that date, and by exhibiting the method of doubling the quaternions to obtain Cayley numbers. He introduced a new imaginary unit {{math|e}}, and for quaternions {{math|q}} and {{math|Q}} writes a Cayley number {{math|q + Qe}}. Denoting the quaternion conjugate by {{math|q′}}, the product of two Cayley numbers is^[5]

The conjugate of a Cayley number is {{math|q' – Qe}}, and the quadratic form is {{math|qq′ + QQ′}}, obtained by multiplying the number by its conjugate. The doubling method has come to be called the Cayley–Dickson construction.

In 1923 the case of real algebras with positive definite forms was delimited by the Hurwitz's theorem (composition algebras).

In 1931 Max Zorn introduced a gamma (γ) into the multiplication rule in the Dickson construction to generate split-octonions.^[6] Adrian Albert also used the gamma in 1942 when he showed that Dickson doubling could be applied to any field with the squaring function to construct binarion, quaternion, and octonion algebras with their quadratic forms.^[7] Nathan Jacobson described the automorphisms of composition algebras in 1958.^[8]

The classical composition algebras over {{math|R}} and {{math|C}} are unital algebras. Composition algebras without a multiplicative identity were found by H.P. Petersson (Petersson algebras) and Susumu Okubo (Okubo algebras) and others.^[9]{{rp|463–81}}

See also

References

1. ^{{cite book | first = T. A. | last = Springer | authorlink = T. A. Springer |author2=F. D. Veldkamp | year = 2000 | title = Octonions, Jordan Algebras and Exceptional Groups | page = 18 | publisher = Springer-Verlag | isbn = 3-540-66337-1}}
2. ^¹Guy Roos (2008) "Exceptional symmetric domains", §1: Cayley algebras, in Symmetries in Complex Analysis by Bruce Gilligan & Guy Roos, volume 468 of Contemporary Mathematics, American Mathematical Society, {{ISBN|978-0-8218-4459-5}}
3. ^{{cite book | first = Richard D. | last = Schafer | year = 1995 | origyear=1966 | zbl=0145.25601 | title = An introduction to non-associative algebras | publisher = Dover Publications | isbn = 0-486-68813-5 | pages = 72–75}}
4. ^¹²Kevin McCrimmon (2004) A Taste of Jordan Algebras, Universitext, Springer {{ISBN|0-387-95447-3}} {{mr|id=2014924}}
5. ^{{Citation | last1=Dickson | first1=L. E. | author1-link=Leonard Dickson | title=On Quaternions and Their Generalization and the History of the Eight Square Theorem | jstor=1967865 | publisher=Annals of Mathematics | series=Second Series | year=1919 | journal=Annals of Mathematics | issn=0003-486X | volume=20 | issue=3 | pages=155–171 | doi=10.2307/1967865}}
6. ^Max Zorn (1931) "Alternativekörper und quadratische Systeme", Abhandlungen aus dem Mathematischen Seminar der Universität Hamburg 9(3/4): 395–402
7. ^{{cite journal | first=Adrian | last=Albert | authorlink=Adrian Albert | year=1942 | title=Quadratic forms permitting composition | journal=Annals of Mathematics | volume=43 | pages=161–177 | zbl=0060.04003 | doi=10.2307/1968887}}
8. ^¹{{cite journal | first=Nathan | last=Jacobson | authorlink=Nathan Jacobson | year=1958 | title=Composition algebras and their automorphisms | journal=Rendiconti del Circolo Matematico di Palermo | volume=7 | pages=55–80 | zbl=0083.02702 | doi=10.1007/bf02854388}}
9. ^Max-Albert Knus, Alexander Merkurjev, Markus Rost, Jean-Pierre Tignol (1998) "Composition and Triality", chapter 8 in The Book of Involutions, pp. 451–511, Colloquium Publications v 44, American Mathematical Society {{ISBN|0-8218-0904-0}}

Structure theorem

Instances and usage

History

See also

References

Further reading