MathSciNet bibliographic data MR2470843 03E55 (03E40) Hamkins, Joel David; Johnstone, Thomas A. The proper and semi-proper forcing axioms for forcing notions that preserve \$\aleph_2\$$\aleph_2$ or \$\aleph_3\$$\aleph_3$. Proc. Amer. Math. Soc. 137 (2009), no. 5, 1823–1833. Article

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