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In a 1977 paper, Steffens identified an elegant criterion for determining when a countable graph has a perfect matching. In this paper, we will investigate the proof-theoretic strength of this result and related theorems. We show that a number of natural variants of these theorems are equivalent, or closely related, to the "big five" subsystems of reverse mathematics. The results of this paper explore the relationship between graph theory and logic by showing the way in which specific changesarXiv:2006.11334v1 fatcat:gnjsgfyngnajbkswztsoua2wdy