Okay kind of a basic one here, just going through the Gibbons book question 1.5,

I'm trying to calculate the both payoffs in the monopoly game

I have my profit function

$\displaystyle \pi = q_i [a - (q_i + q_j) - c] $

And my quantities in monopoly are qm - after maximising profits

$\displaystyle q_i * = (a + q_j* - c )/2 \textnormal{and} q_j * = (a + q_i* - c )/2 $

since in monopoly

$\displaystyle (q_m / 2) = q_i = q_j = [((a-c)/2) / 2] = [(a-c)/4] $

my profits are (a-c)^2 / 4

which is wrong and should be divided by 8 - not sure where I have gone wrong...