In case you're curious, the five systems in the article are mentioned by the original paper, and correspond to the ones described here: https://en.wikipedia.org

In case you're curious, the five systems in the article are mentioned by the original paper, and correspond to the ones described here: https://en.wikipedia.org/wiki/Reverse_mathematics#The_big_five_subsystems_of_second_order_arithmetic https://en.wikipedia.org/wiki/Reverse_mathematics#The_big_fi...