∫-∞∞Hℓ⁡(x)⁢Hm⁡(x)⁢Hn⁡(x)⁢ⅇ-x2⁢ⅆ⁡x=212⁢(ℓ+m+n)⁢ℓ!⁢m!⁢n!⁢π(12⁢ℓ+12⁢m-12⁢n)!⁢(12⁢m+12⁢n-12⁢ℓ)!⁢(12⁢n+12⁢ℓ-12⁢m)!,