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Is there an elementary example of a function f, such that $|f(x+t)+f(x-t)-2f(x)|/|x|^a\le C$$|f(x+t)+f(x-t)-2f(x)|/|t|^a\le C$, where $a>1$, such that $f$ is not $C^1$?
Is there an elementary example of a function f, such that $|f(x+t)+f(x-t)-2f(x)|/|x|^a\le C$, where $a>1$, such that $f$ is not $C^1$?
Is there an elementary example of a function f, such that $|f(x+t)+f(x-t)-2f(x)|/|t|^a\le C$, where $a>1$, such that $f$ is not $C^1$?
