(adsbygoogle = window.adsbygoogle || []).push({}); What is "Euler's Criterion" in plain simplified english

This sentence in textbook reads: "If ∂M/∂y = ∂N/∂x the differential is exact (Euler’s Criterion)."

What does the equation mean, and also im not familiar with the backwards number 6 symbol --what is that?

thanks

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# What is Euler's Criterion in plain simplified english

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