12⁢π⁢∫-∞∞ⅇ-ⅈ⁢a⁢t⁢(∫-∞∞ϕ⁡(x)⁢ⅇⅈ⁢t⁢x⁢ⅆ⁡x)⁢ⅆ⁡t=ϕ⁡(a)