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Fredholm determinant : ウィキペディア英語版
Fredholm determinant
In mathematics, the Fredholm determinant is a complex-valued function which generalizes the determinant of a matrix. It is defined for bounded operators on a Hilbert space which differ from the identity operator by a trace-class operator. The function is named after the mathematician Erik Ivar Fredholm.
Fredholm determinants have had many applications in mathematical physics, the most celebrated example being Gábor Szegő's limit formula, proved in response to a question raised by Lars Onsager and C. N. Yang on the spontaneous magnetization of the Ising model.
==Definition==
Let ''H'' be a Hilbert space and ''G'' the set of bounded invertible operators on ''H'' of the form ''I'' + ''T'', where ''T'' is a trace-class operator. ''G'' is a group because
: (I+T)^ - I = - T(I+T)^,
so ''(I+T)-1-I'' is trace class if ''T'' is. It has a natural metric given by ''d''(''X'', ''Y'') = ||''X'' - ''Y''||1, where || · ||1 is the trace-class norm.
If ''H'' is a Hilbert space with inner product (\cdot,\cdot), then so too is the ''k''th exterior power \Lambda^k H with inner product
: (v_1 \wedge v_2 \wedge \cdots \wedge v_k, w_1 \wedge w_2 \wedge \cdots \wedge w_k) = \, (v_i,w_j).
In particular
: e_ \wedge e_ \wedge \cdots \wedge e_, \qquad (i_1
gives an orthonormal basis of \Lambda^k H if (''e''''i'') is an orthonormal basis of ''H''.
If ''A'' is a bounded operator on ''H'', then ''A'' functorially defines a bounded operator \Lambda^k(A)
on \Lambda^k H by
: \Lambda^k(A) v_1 \wedge v_2 \wedge \cdots \wedge v_k = Av_1 \wedge Av_2 \wedge \cdots \wedge Av_k.
If ''A'' is trace-class, then \Lambda^k(A) is also trace-class with
: \|\Lambda^k(A)\|_1 \le \|A\|_1^k/k!.
This shows that the definition of the Fredholm determinant given by
: \, (I+ A) = \sum_^\infty \Lambda^k(A)
makes sense.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
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