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antisymmetrizer : ウィキペディア英語版
antisymmetrizer
In quantum mechanics, an antisymmetrizer \mathcal (also known as antisymmetrizing operator〔P.A.M. Dirac, ''The Principles of Quantum Mechanics'', 4th edition, Clarendon, Oxford UK, (1958) p. 248〕 ) is a linear operator that makes a wave function of ''N'' identical fermions antisymmetric under the exchange of the coordinates of any pair of fermions. After application of \mathcal the wave function satisfies the Pauli principle. Since \mathcal is a projection operator,
application of the antisymmetrizer to a wave function that is already totally antisymmetric has no effect, acting as the identity operator.
==Mathematical definition ==
Consider a wave function depending on the space and spin coordinates of ''N'' fermions:
:
\Psi(1,2, \ldots, N)\quad\text \quad i \leftrightarrow (\mathbf_i, \sigma_i),

where the position vector r''i'' of particle ''i'' is a vector in \mathbb^3 and σi takes on 2''s''+1 values, where ''s'' is the half-integral intrinsic spin of the fermion. For electrons ''s'' = 1/2 and σ can have two values ("spin-up": 1/2 and "spin-down": −1/2). It is assumed that the positions of the coordinates in the notation for Ψ have a well-defined meaning. For instance, the 2-fermion function Ψ(1,2) will in general be not the same as Ψ(2,1). This implies that in general \Psi(1,2)- \Psi(2,1) \ne 0 and therefore we can define meaningfully a ''transposition operator'' \hat_ that interchanges the coordinates of particle ''i'' and ''j''. In general this operator will not be equal to the identity operator (although in special cases it may be).
A transposition has the
parity (also known as signature) −1. The Pauli principle postulates that a wave function of identical fermions must be an eigenfunction of a transposition operator with its parity as eigenvalue
:
\begin
\hat_ \Psi\big(1,2,\ldots,i, \ldots,j,\ldots, N\big)& \equiv \Psi\big(\pi(1),\pi(2),\ldots,\pi(i), \ldots,\pi(j),\ldots, \pi(N)\big) \\
&\equiv \Psi(1,2,\ldots,j, \ldots,i,\ldots, N) \\
&= - \Psi(1,2,\ldots,i, \ldots,j,\ldots, N).
\end

Here we associated the transposition operator \hat_ with the permutation of coordinates ''π'' that acts on the set of ''N'' coordinates. In this case ''π'' = (''ij''), where (''ij'') is the cycle notation for the transposition of the coordinates of particle ''i'' and ''j''.
Transpositions may be composed (applied in sequence). This defines a product between the transpositions that is associative.
It can be shown that an arbitrary permutation of ''N'' objects can be written as a product of transpositions and that the number of transposition in this decomposition is of fixed parity. That is, either a permutation is always decomposed in an even number of transpositions (the permutation is called even and has the parity +1), or a permutation is always decomposed in an odd number of transpositions and then it is an odd permutation with parity −1. Denoting the parity of an arbitrary permutation ''π'' by (−1)''π'', it follows that an antisymmetric wave function satisfies
:
\hat \Psi\big(1,2,\ldots, N\big) \equiv \Psi\big(\pi(1),\pi(2),\ldots, \pi(N)\big) = (-1)^\pi \Psi(1,2,\ldots, N),

where we associated the linear operator \hat with the permutation π.
The set of all ''N''! permutations with the associative product: "apply one permutation after the other", is a group, known as the permutation group or symmetric group, denoted by ''S''''N''. We define the antisymmetrizer as
:
\mathcal \equiv \frac \sum_ (-1)^\pi \hat .


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