1k!⁢f(k)⁢(x0)=12⁢π⁢rk⁢∫02⁢πf⁢(x0+r⁢ⅇⅈ⁢θ)⁢ⅇ-ⅈ⁢k⁢θ⁢ⅆ⁡θ.