 Call price in an arithmetic stock model with interest

Let $\nu =\sigma \sqrt{(1-e^{-2r\tau })/(2r)}$ and $d=(s-K{e}^{-r\tau })/\nu$, where $\tau =T-t$. The displayed discounted Gaussian positive-part expectation prices a call in the arithmetic stock model with constant volatility. Its delta is $\Phi (d)$. The stock holding $C_s$ and bank-account holding $(C-s{C}_s)/B$ replicate the payoff with nonnegative wealth. A nonnegative discounted-wealth supermartingale bound proves this is the least replication capital.