○○○5.2.3 Left and Right Theorem⪢⪢
f is continuous at c if and only if f is both left-continuous and right-continuous at c.
If ⎾lim f(x)⏐x￫c⁻⏌ exists and equals f(c) then f is left-continuous at c. If ⎾lim f(x)⏐x￫c⁺⏌ exists and equals f(c) then f is right-continuous at c.
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