We prove that the function (g(x)= 1 / ( 1 - cos(x) )) is completely monotonic on ((0,pi]) and absolutely monotonic on ([pi, 2pi)), and we determine the best possible bounds (lambda_n) and (mu_n) such that the inequalities
$$ lambda_n leq g^{(n)}(x)+g^{(n)}(y)-g^{(n)}(x+y) quad (n geq 0 mbox{even}) $$