1. For a recent review: P. Fulde in “Electron Correlations in Molecules and Solids”, Springer Series in Solid State Sciences, vol. 100, (Springer Berlin 1992).
2. We use notation such that σ = ↑, ↓ is the spin, x and y specify sites in the d-dimensional lattice A, and h
x,y is the hopping probability from site x to
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y;c_{x\sigma }^ +
$$
\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle-}$}}{c}
$$
and c
yσ are standard fermion creation and annihilation operators,
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$$
{n_{x\sigma }} = c_{x\sigma }^ + {c_{x\sigma }}
$$
is the local occupation number of one particle states with spin σ, and
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$$
{D_x} = {n_{x \uparrow }}{n_{x \uparrow }}
$$
is the double occupancy at site x.
3. For example: K.G. Wilson, Rev. Mod. Phys. 47, 773 (1975).
4. A.B. Harris and R.V. Lange, Phys. Rev. 157, 295 (1967)
5. A.H. MacDonald, S.M. Girvin und D. Yoshioka, Phys. Rev. B37, 9753 (1988).