for the case ak = b for k < m and at = a for k m with a > b. I assume again that all dt = d. In this case, the left hand side of (1.4) reads 9 v Leisn ( N+ 41I))11 ( 41:1:137LOV II+0, (1.10) Evaluating these sums gives the instability condition r -i I (Sri > (qa+b) b (q+ I) I -q ' c) ak is