(1)

\begin{align*} H\left(\pm1\right) & =\frac{\sgn\left(\pm1\right)+1}{2}\\ & =\frac{1\pm1}{2} \end{align*}

(1)-2

\begin{align*} H\left(\pm1\right) & =\begin{cases} 1 & \pm1\rightarrow+1\\ 0 & \pm1\rightarrow-1 \end{cases}\\ & =\frac{1\pm1}{2} \end{align*}

(2)

\begin{align*} H\left(\pm1\right)+H\left(\mp1\right) & =\frac{1\pm1}{2}+\frac{1\mp1}{2}\\ & =1 \end{align*}

(3)

\begin{align*} H\left(\pm1\right)-H\left(\mp1\right) & =\frac{1\pm1}{2}-\frac{1\mp1}{2}\\ & =\pm1 \end{align*}