genmerally we know that linear system is defined by following two rule
$1.T(x+y)=T(x)+T(y);$
$2.T(c*x)=c*T(x) $
or operation on sum of two income is equal to sum of operation on each input and also operation on input multiplied some constant is equation constant multiplied by operation on given input,now let us consider following model
$$ x(n) = \sum_{p=1}^{P} a_p \cos(2\pi f_pn + \phi_p) + \epsilon(n) $$
clearly $cos(a+b) \neq cos(a)+cos(b)$ and also $cos(c*a) \neq c*cos(a)$,so that means that system driven by sinusoidal/cosine model is not linear right?thanks in advance