Wednesday, February 27, 2019

limits - f(x+y)=f(x)+f(y) continuous at x0=0impliesf(x) is continuous over R?


Let x,yR


f(x+y)=f(x)+f(y)


is it true that if f is continuous at x0=0, than f is continuous in R?


Answer




At any arbitrary x1R and any Δ0, we have f(x1+Δ)f(x1)=f(Δ)=f(Δ+x0)f(x0).

As Δ0, the rightmost expression above goes to 0 due to continuity at x0, so the leftmost expression also goes to 0. This implies continuity at x1 and therefore in R. Note we don't need the fact that x0=0.


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