1. Astier V. and Tressl M. (2005). Axiomatization of local-global principles for pp-formulas in spaces of orderings. Arch. Math. Logic 44(1): 77–95
2. Dickmann, M.: Anneaux de witt abstraits et groupes spéciaux. In: Séminaire de Structures Algébriques Ordonnées, vol. 42, Paris VII-CNRS, Logique, Prépublications (1993)
3. Dickmann M.A. and Miraglia F. (2000). Special groups: Boolean-theoretic methods in the theory of quadratic forms (With appendixes A and B by Dickmann and A. Petrovich). Mem. Am. Math. Soc. 145(689): xvi+247
4. Ellerman D.P. (1974). Sheaves of structures and generalized ultraproducts. Ann. Math. Logic 7: 163–195
5. Gladki, P., Marshall, M.: The pp conjecture for spaces of orderings of rational conics (to appear)