Π₂¹ sets and Π₂¹ singletons
Π₂¹ sets and Π₂¹ singletons
The following are equivalent: (a) every real is constructible; (b) every nonempty <inline-formula content-type="math/mathml"> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="product Underscript 2 Overscript 1 Endscripts"> <mml:semantics> <mml:munderover> <mml:mo>∏</mml:mo> <mml:mn>2</mml:mn> <mml:mn>1</mml:mn> </mml:munderover> <mml:annotation encoding="application/x-tex">\prod _2^1</mml:annotation> </mml:semantics> </mml:math> </inline-formula> set of reals contains a <inline-formula content-type="math/mathml"> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" alttext="product Underscript 2 Overscript 1 Endscripts"> <mml:semantics> …