Sentence:  The equivalence theorem: "If both M is <WSD>equivalent</WSD> to a subset N<sub>1</sub> of N and N is <WSD>equivalent</WSD> to a subset M<sub>1</sub> of M, then the sets M and N are <WSD>equivalent</WSD> to each other." 
Meaning of the word marked with <WSD> tags:  Of two sets, having a one-to-one correspondence; equinumerous.