I have the following Question :
Prove that under the risk-neutral probability p the stock and the banjaccount have the same average rate of growth. In other words, if $ S_0 , S_N $ are the initial and final stock prices and $B_0 , B_N $ the initial and final bank prices , show that :
$$ E[S_N / S_0 ] = E[B_N / B_0 ] = c $$
Hint : The discounted stock price is a martingale under P.
Could you explain to me what is the discounted stock price ?