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Proposizione 36

[0BH] Sia \(I\) un insieme, \(x_ 0βˆˆβ„\) punto di accumulazione per \(I\), \(f:Iβ†’ ℝ\) funzione, \(lβˆˆβ„\).

Mettendo insieme tutte le definizioni viste precedentemente, otteniamo queste definizioni di limite.

Nel caso \(x_ 0βˆˆβ„\) e \(lβˆˆβ„\):

\(\lim _{x→ x_ 0} f(x) = l\)

\(βˆ€ \varepsilon {\gt}0, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, xβ‰  x_ 0, x∈ Iβ‡’|f(x)-l|{\lt}\varepsilon \)

\(\lim _{x→ x_ 0^+} f(x) = l\)

\(βˆ€ \varepsilon {\gt}0, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\gt} x_ 0, x∈ Iβ‡’|f(x)-l|{\lt}\varepsilon \)

\(\lim _{x→ x_ 0^-} f(x) = l\)

\(βˆ€ \varepsilon {\gt}0, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\lt} x_ 0, x∈ Iβ‡’|f(x)-l|{\lt}\varepsilon \)

Sia \(x_ 0βˆˆβ„\), \(l=±∞\).

\(\lim _{xβ†’ x_ 0} f(x) = ∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, xβ‰  x_ 0, x∈ Iβ‡’f(x){\gt}z \)

\(\lim _{xβ†’ x_ 0} f(x) = -∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, xβ‰  x_ 0, x∈ Iβ‡’f(x){\lt}z \)

\(\lim _{xβ†’ x_ 0^+} f(x) = ∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\gt} x_ 0, x∈ Iβ‡’f(x){\gt}z \)

\(\lim _{xβ†’ x_ 0^+} f(x) = -∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\gt} x_ 0, x∈ Iβ‡’f(x){\lt}z \)

\(\lim _{xβ†’ x_ 0^-} f(x) = ∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\lt} x_ 0, x∈ Iβ‡’f(x){\gt}z \)

\(\lim _{xβ†’ x_ 0^-} f(x) = -∞\)

\(βˆ€ z, βˆƒ 𝛿 {\gt}0, βˆ€ x, |x-x_ 0|{\lt}𝛿, x{\lt} x_ 0, x∈ Iβ‡’f(x){\lt}z \)

Sia \(lβˆˆβ„\), \(x_ 0=±∞\).

\(\lim _{xβ†’ ∞} f(x) = l\)

\(βˆ€ \varepsilon {\gt}0, βˆƒ y, βˆ€ x, x{\gt}y, x∈ Iβ‡’|f(x)-l|{\lt}\varepsilon \)

\(\lim _{xβ†’ -∞} f(x) = l\)

\(βˆ€ \varepsilon {\gt}0, βˆƒ y, βˆ€ x, x{\lt}y, x∈ Iβ‡’|f(x)-l|{\lt}\varepsilon \)

\(\lim _{xβ†’ ∞} f(x) = ∞\)

\(βˆ€ z, βˆƒ y, βˆ€ x, x{\gt}y, x∈ Iβ‡’f(x){\gt}z\)

\(\lim _{xβ†’ -∞} f(x) = ∞\)

\(βˆ€ z, βˆƒ y, βˆ€ x, x{\lt}y, x∈ Iβ‡’f(x){\gt}z\)

\(\lim _{xβ†’ ∞} f(x) = -∞\)

\(βˆ€ z, βˆƒ y, βˆ€ x, x{\gt}y, x∈ Iβ‡’f(x){\lt}z\)

\(\lim _{xβ†’ -∞} f(x) = -∞\)

\(βˆ€ z, βˆƒ y, βˆ€ x, x{\lt}y, x∈ Iβ‡’f(x){\lt}z\)

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