He means "underconstrained". In other words, there is more than one function that satisfies the relation f(2x) = f(x) * f(x), so we need to find some more cons

He means "underconstrained". In other words, there is more than one function that satisfies the relation f(2x) = f(x) * f(x), so we need to find some more constraints to exactly pin down the Maclaurin expansion of exp(x).