The relative strength of two systems of formal logic can be defined via model theory. Specifically, a logic [math]\displaystyle{ \alpha }[/math] is said to be as strong as a logic [math]\displaystyle{ \beta }[/math] if every elementary class in [math]\displaystyle{ \beta }[/math] is an elementary class in [math]\displaystyle{ \alpha }[/math].[1]
Original source: https://en.wikipedia.org/wiki/Strength (mathematical logic).
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