Julia | Math & Programming on Instagram: "Flip them around, Fubini-Tonelli: Let X, µ and Y, ν be measure spaces with corresponding σ-algebras A, B. Assume X and Y are σ-finite. Theorem: (Fubini-Tonelli) For complex-valued measurable f, g, if any one of ∫_X ∫_Y |f(x, y)| dµ(x) dν(y) ∫_Y ∫_X |f(x, y)| dν(y) dµ(x) ∫_X×Y |f(x, y)| dµ×ν is finite, then they all are finite, and are equal. For [0, +∞]-valued functions f, ∫_X ∫_Y f(x, y) dµ(x) dν(y) = ∫_Y ∫_X f(x, y) dν(y) dµ(x) = ∫_X×Y f(x, y) dµ×ν alt