Let $\left\{a_k\right\}_{0\leqslant k\leqslant n}$ and $\left\{b_k\right\}_{0\leqslant k\leqslant n}$ be two positive sequences such that $c_1a_k\leqslant b_k\leqslant c_2 a_k$ for all $k$ for some positive constants $c_1,c_2$. Let $\epsilon>0$ and $f$ a function such that $$\sum_{k=0}^na_k\sin(k\epsilon)\geqslant f(n).$$ Then is it true that $$\sum_{k=0}^nb_k\sin(k\epsilon)\geqslant cf(n)$$ for some constant $c$ (independent of $n$, $\epsilon$)? Intuitively I would say yes. What do you think?