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Published Aug 08, 2023

If Y1Y_1 is a standard normal random variable, then Y12Y_1^2 will follow a Chi-square distribution with 1 degree of freedom

It can be observed that Y1Y_1 has a range of −1≤Y1≤1-1 \leq Y_1 \leq 1. Similarly, Y2Y_2 is 0≤Y2≤10 \leq Y_2 \leq 1. To form the triangle shape, an additional constraint is needed: ∣Y1∣+Y2≤1|Y_1| + Y_2 \leq 1.

f(y1,y2)={1−1≤Y1≤1,0≤Y2≤1,∣Y1∣+Y2≤10otherwise\begin{equation*} f(y_1,y_2) = \begin{cases} 1 & -1 \leq Y_1 \leq 1, 0 \leq Y_2 \leq 1, |Y_1| + Y_2 \leq 1 \\ 0 & \textnormal{otherwise} \end{cases} \end{equation*}

A split integral will be needed to find E(Y1Y2)E(Y_1Y_2) due to the special constraint.

E(Y1Y2)=∫−10∫01+y1y1y2dy2dy1+∫01∫01−y1y1y2dy2dy1=∫−10y1(1+y1)22dy1+∫01y1(1−y1)22dy1=12∫−10y1(1+y1)2dy1+12∫01y1(1−y1)2dy1=12∫−10(y1+y13+2y12)dy1+12∫01(y1+y13−2y12)dy1 (expanded)=12[y122+y144+2y133]−10+12[y122+y144−2y133]01=−12[12+14−23]+12[12+14−23]=0\begin{align*} E(Y_1Y_2) &= \int_{-1}^{0}\int_{0}^{1+y_1}y_1y_2dy_2dy_1 + \int_{0}^{1}\int_{0}^{1-y_1}y_1y_2dy_2dy_1 \\ &= \int_{-1}^{0}\dfrac{y_1(1+y_1)^2}{2}dy_1 + \int_{0}^{1}\dfrac{y_1(1-y_1)^2}{2}dy_1 \\ &= \dfrac{1}{2}\int_{-1}^{0}y_1(1+y_1)^2dy_1 + \dfrac{1}{2} \int_{0}^{1}y_1(1-y_1)^2dy_1 \\ &= \dfrac{1}{2}\int_{-1}^{0}\left(y_1+y_1^3+2y_1^2\right)dy_1 + \dfrac{1}{2}\int_{0}^{1}\left(y_1+y_1^3-2y_1^2\right)dy_1 \textnormal{ (expanded)} \\ &= \dfrac{1}{2}\left[ \dfrac{y_1^2}{2} + \dfrac{y_1^4}{4} + \dfrac{2y_1^3}{3} \right]_{-1}^0 + \dfrac{1}{2}\left[ \dfrac{y_1^2}{2} + \dfrac{y_1^4}{4} - \dfrac{2y_1^3}{3} \right]_0^1 \\ &= -\cancel{\dfrac{1}{2}\left[ \dfrac{1}{2} + \dfrac{1}{4} - \dfrac{2}{3} \right]} + \cancel{\dfrac{1}{2}\left[ \dfrac{1}{2} + \dfrac{1}{4} - \dfrac{2}{3} \right]} \\ &= 0 \end{align*}

The uniqueness property of mgfs allows us to show that UU follows a Chi-square distribution with 2 degrees of freedom.