Let x,y∈R
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 x1∈R and any Δ≠0, we have f(x1+Δ)−f(x1)=f(Δ)=f(Δ+x0)−f(x0).
As Δ→0, 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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