PDF of Lecture Notes - School of Mathematical Sciences
PDF of Lecture Notes - School of Mathematical Sciences
PDF of Lecture Notes - School of Mathematical Sciences
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1. DISTRIBUTION THEORY<br />
Just substitute<br />
P (x, y − x) = P X1 (x)P X2 (y − x).<br />
Theorem. 1.8.2<br />
Suppose X 1 , X 2 are continuous with <strong>PDF</strong>, f(x 1 , x 2 ), and let Y = X 1 + X 2 . Then<br />
1. f Y (y) =<br />
∫ ∞<br />
−∞<br />
f(x, y − x) dx.<br />
2. If X 1 , X 2 are independent, then<br />
f Y (y) =<br />
Pro<strong>of</strong>. 1. F Y (y) = P (Y ≤ y)<br />
∫ ∞<br />
−∞<br />
= P (X 1 + X 2 ≤ y)<br />
f X1 (x)f X2 (y − x) dx.<br />
=<br />
∫ ∞ ∫ y−x1<br />
x 1 =−∞ x 2 =−∞<br />
f(x 1 , x 2 ) dx 2 dx 1 .<br />
Let x 2 = t − x 1 ,<br />
⇒ dx 2<br />
dt = 1<br />
⇒ dx 2 = dt<br />
=<br />
=<br />
∫ ∞ ∫ y<br />
−∞<br />
∫ y<br />
−∞<br />
−∞<br />
f(x 1 , t − x 1 ) dt dx 1<br />
{∫ ∞<br />
}<br />
f(x 1 , t − x 1 ) dx 1 dt<br />
−∞<br />
⇒ f Y (y) = F ′ Y (y) =<br />
∫ ∞<br />
−∞<br />
f(x, y − x) dx.<br />
Pro<strong>of</strong>. (2)<br />
Take f(x, y − x) = f X1 (x) f X2 (y − x) if X 1 , X 2 independent.<br />
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