$\frac{2}{\sqrt{2\pi}} e^{-y^2/2}$ for $y > 0$

$\sqrt{\frac{2}{\pi}} y e^{-y^2/2}$ for $y > 0$

$\frac{1}{\sqrt{2\pi}} e^{-y^2/2}$ for $y > 0$

$\sqrt{\frac{2}{\pi}} e^{-y^2/2}$ for $y > 0$