- Indefinite inner product space
In
mathematics , in the field offunctional analysis , an indefinite inner product space:K, langle cdot,,cdot angle, J)
is an infinite-dimensional complex
vector space K equipped with both an indefiniteinner product :langle cdot,,cdot angle
and a positive semi-definite inner product
:x,,y) stackrel{mathrm{def{=} langle x,,Jy angle,
where the
metric operator J is an endomorphism of K obeying:J^3 = J.
The indefinite inner product space itself is not necessarily a
Hilbert space ; but the existence of a positive semi-definite inner product on K implies that one can form aquotient space on which there is a positive definite inner product. Given a strong enoughtopology on this quotient space, it has the structure of a Hilbert space, and many objects of interest in typical applications fall into this quotient space.An indefinite inner product space is called a Krein space (or J"-space") if x,,y) is positive definite and K possesses a
majorant topology . Krein spaces are named in honor of the Ukrainian mathematicianMark Grigorievich Krein (3 April 1907 -17 October 1989 ).Inner products and the metric operator
Consider a complex
vector space K equipped with an indefinitehermitian form langle cdot ,, cdot angle. In the theory of Krein spaces it is common to call such ahermitian form an indefinite inner product. The following subsets are defined in terms of thesquare norm induced by the indefinite inner product::K_{0} stackrel{mathrm{def{=} { x in K : langle x,,x angle = 0 } ("neutral"):K_{++} stackrel{mathrm{def{=} { x in K : langle x,,x angle > 0 } ("positive"):K_{--} stackrel{mathrm{def{=} { x in K : langle x,,x angle < 0 } ("negative"):K_{+0} stackrel{mathrm{def{=} K_{++} cup K_{0} ("non-negative"):K_{-0} stackrel{mathrm{def{=} K_{--} cup K_{0} ("non-positive")
A
subspace L subset K lying within K_{0} is called a "neutral subspace". Similarly, a subspace lying within K_{+0} (K_{-0}) is called "positive" ("negative") "semi-definite", and a subspace lying within K_{++} cup {0} (K_{--} cup {0}) is called "positive" ("negative") "definite". A subspace in any of the above categories may be called "semi-definite", and any subspace that is not semi-definite is called "indefinite".Let our indefinite inner product space also be equipped with a decomposition into a pair of subspaces K = K_+ oplus K_-, called the "fundamental decomposition", which respects the complex structure on K. Hence the corresponding linear projection operators P_pm coincide with the identity on K_pm and annihilate K_mp, and they commute with multiplication by the i of the complex structure. If this decomposition is such that K_+ subset K_{+0} and K_- subset K_{-0}, then K is called an indefinite inner product space; if K_pm subset K_{pmpm} cup {0}, then K is called a Krein space, subject to the existence of a
majorant topology on K.The operator J stackrel{mathrm{def{=} P_+ - P_- is called the (real phase) "metric operator" or "fundamental symmetry", and may be used to define the "Hilbert inner product" cdot,,cdot):
:x,,y) stackrel{mathrm{def{=} langle x,,Jy angle = langle x,,P_+ y angle - langle x,,P_- y angle
On a Krein space, the Hilbert inner product is positive definite, giving K the structure of a Hilbert space (under a suitable topology). Under the weaker constraint K_pm subset K_{pm0}, some elements of the neutral subspace K_0 may still be neutral in the Hilbert inner product, but many are not. For instance, the subspaces K_0 cap K_pm are part of the neutral subspace of the Hilbert inner product, because an element k in K_0 cap K_pm obeys k,,k) stackrel{mathrm{def{=} langle k,,Jk angle = pm langle k,,k angle = 0. But an element k = k_+ + k_- (k_pm in K_pm) which happens to lie in K_0 because langle k_-,,k_- angle = - langle k_+,,k_+ angle will have a positive square norm under the Hilbert inner product.
We note that the definition of the indefinite inner product as a Hermitian form implies that:
:langle x,,y angle = frac{1}{4} (langle x+y,,x+y angle - langle x-y,,x-y angle)
Therefore the indefinite inner product of any two elements x,,y in K which differ only by an element x-y in K_0 is equal to the square norm of their average frac{x+y}{2}. Consequently, the inner product of any non-zero element k_0 in (K_0 cap K_pm) with any other element k_pm in K_pm must be zero, lest we should be able to construct some k_pm + 2 lambda k_0 whose inner product with k_pm has the wrong sign to be the square norm of k_pm + lambda k_0 in K_pm.
Similar arguments about the Hilbert inner product (which can be demonstrated to be a Hermitian form, therefore justifying the name "inner product") lead to the conclusion that its neutral space is precisely K_{00} = (K_0 cap K_+) oplus (K_0 cap K_-), that elements of this neutral space have zero Hilbert inner product with any element of K, and that the Hilbert inner product is positive semi-definite. It therefore induces a positive definite inner product (also denoted cdot,,cdot)) on the quotient space ilde{K} stackrel{mathrm{def{=} K / K_{00}, which is the direct sum of ilde{K}_pm stackrel{mathrm{def{=} K_pm / (K_0 cap K_pm). Thus ilde{K},,(cdot,,cdot)) is a
Hilbert space (given a suitable topology).Properties and applications
Krein spaces arise naturally in situations where the indefinite inner product has an analytically useful property (such as
Lorentz invariance ) which the Hilbert inner product lacks. It is also common for one of the two inner products, usually the indefinite one, to be globally defined on a manifold and the other to be coordinate-dependent and therefore defined only on a local section.In many applications the positive semi-definite
inner product cdot,,cdot) depends on the chosen fundamental decomposition, which is, in general, not unique. But it may be demonstrated (e. g., cf. Proposition 1.1 and 1.2 in the paper of H. Langer below) that any two metric operators J and J^prime compatible with the same indefinite inner product on K result in Hilbert spaces ilde{K} and ilde{K}^prime whose decompositions ilde{K}_pm and ilde{K}^prime_pm have equal dimensions. Although the Hilbert inner products on these quotient spaces do not generally coincide, they induce identical square norms, in the sense that the square norms of the equivalence classes ilde{k} in ilde{K} and ilde{k}^prime in ilde{K}^prime into which a given k in K falls are equal. All topological notions in a Krein space, likecontinuity ,closed -ness of sets, and thespectrum of anoperator on ilde{K}, are understood with respect to this Hilbert spacetopology .Isotropic part and degenerate subspaces
Let L, L_{1}, L_{2} be subspaces of K. The
subspace L^{ [perp] } stackrel{mathrm{def{=} { x in K : langle x,,y angle = 0 for all y in L } is called the orthogonal companion of L, and L^{0} stackrel{mathrm{def{=} L cap L^{ [perp] } is the isotropic part of L. If L^{0} = {0}, L is called non-degenerate; otherwise it is degenerate. If langle x,,y angle = 0 for all x in L_{1},,, y in L_{2}, then the two subspaces are said to be orthogonal, and we write L_{1} [perp] L_{2}. If L = L_{1} + L_{2} where L_{1} [perp] L_{2}, we write L = L_{1} [+] L_{2}. If, in addition, this is adirect sum , we write L= L_{1} [dot{+}] L_{2}.Pontrjagin space
If kappa := min { dim K_{+}, dim K_{-} } < infty, the Krein space K, langle cdot,,cdot angle, J) is called a Pontrjagin space or Pi_{kappa}-space. (Conventionally, the indefinite inner product is given the sign that makes dim K_{+} finite.) In this case dim K_{+} is known as the "number of positive squares" of langle cdot,,cdot angle. Pontrjagin spaces are named after
Lev Semenovich Pontryagin .Literature
* Bognár, J. : "Indefinite inner product spaces", Springer-Verlag, Berlin-Heidelberg-New York, 1974, ISBN 3-540-06202-5.
* Springer "Encyclopaedia of Mathematics" entry for "Krein space", contributed by H. Langer (http://eom.springer.de/k/k055840.htm)
* Azizov, T.Ya.; Iokhvidov, I.S. : "Linear operators in spaces with an indefinite metric", John Wiley & Sons, Chichester, 1989, ISBN 0-471-92129-7.
* Langer, H. : "Spectral functions of definitizable operators in Krein spaces", Functional Analysis Proceedings of a conference held at Dubrovnik, Yugoslavia, November 2-14, 1981, Lecture Notes in Mathematics, 948, Springer-Verlag Berlin-Heidelberg-New York, 1982, 1-46, ISSN 0075-8434.References
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