Round 400: Tossup 11

In 2020, Scott Aaronson conjectured that the busy beaver function of 20 was the upper bound of what this system could prove. Shelah showed that two implications of this system (10[1])produce contradictory answers to the Whitehead problem. The first-order form (10[1])of this system violates the Löwenheim–Skolem theorem, leading to Skolem’s paradox. In 1963, Paul Cohen used forcing to show that this system and the continuum (10[1])hypothesis are independent of each other. This system’s inclusion (10[1])of the axiom of specification allows it to avoid (10[1])Russell’s paradox. The letter C is appended to this system’s name to indicate its inclusion of the axiom of choice. For 10 points, name this standard formulation of set theory proposed by two German mathematicians, often known by a two-letter acronym. (10[1])■END■

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