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Reference.org
Proper forcing axiom
open-in-new
<p>In the mathematical field of <a href="/facts/Set_theory/1ptfsvUP">set theory</a>, the proper forcing axiom (<i>PFA</i>) is a significant strengthening of <a href="/facts/Martin%2527s_axiom/TvtNNj1o">Martin's axiom</a>, where <a href="/facts/Forcing_(set_theory)/3vMqqKcq">forcings</a> with the <a href="/facts/Countable_chain_condition/6awtjEdY">countable chain condition</a> (ccc) are replaced by proper forcings. </p>