Faizrahmanov M.; Kalimullin I.; Montalbán A.; Puzarenko V.
(2017)
© J.UCS. Studying the Σ-reducibility of families introduced by [Kalimullin and Puzarenko 2009] we show that for every set X ≥ T ∅׳_there is a family of sets F which is the Σ-least countable family whose Σ-jump is Σ-equivalent ...