โ <i>CA</i><sub>0</sub> and order types of countable ordered groups
โ <i>CA</i><sub>0</sub> and order types of countable ordered groups
Reverse mathematics uses subsystems of second order arithmetic to determine which set existence axioms are required to prove particular theorems. Surprisingly, almost every theorem studied is either provable in RCA 0 or equivalent over RCA 0 to one of four other subsystems: WKL 0 , ACA 0 , ATR 0 โฆ