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I came across the following sentence ( from "Applied Nonlinear Control book by SLOTINE"

"An important concept in phase plane analysis is that of a singular point."

It seems to me that saying "... is the singular point " is clearer that saying "... is that of a singular point". Am I correct? and what does "that of" in this context mean?

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That is a pronoun, standing for an important concept. The sentence says (or was intended to say) that singular points are an important concept in phase plane analysis: “An important concept in phase plane analysis is the concept of singular points.”

No particular singular point is being singled out here (a function may have many singular points), so a definite article (as in the singular point) is not appropriate. Indefinite article a is acceptable, but using a plural form, singular points, is better.

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