T(\langle x_k \rangle )= \sum_{k=1}^\infty |a_k x_k| = \sum_{k=1}^\infty |a_k| \cdot |x_k|=\sum_{k=1}^\infty |a_k| \cdot |y_k|=\sum_{k=1}^\infty |a_k y_k|=T(\langle y_k \rangle )