The characteristic equation of a T flip-flop is : The symbols used have the usual meaning.

A T flip-flop holds its state when T = 0 and toggles when T = 1, so its next state is $Q_{n+1} = T \oplus Q_n = T\bar{Q}_n + \bar{T}Q_n$. The expression T + Qₙ fails because with T = 1 it always gives 1 instead of toggling, and TQₙ fails because with T = 0 it gives 0 instead of holding.