For a one dimensional half-infinite chain with diagonal disorder we calculated the ultimate at
t→∞ value of the average excitation density at the edge site
D if at
t=0 the excitation was localised at the edge site (Anderson' s creterium). We obtained the following results: i) for the binary disordered chain we derived the close expression for
D which is exact in the limit of low concentration of defects and is valid for an arbitrary energy of defects
ε. In this case
D demonstrated the non analytical dependence on
ε. ii) The close expression for
D is obtained for the case of an arbitrary small disorder. iii) The relative contribution of states with specified energy to
D is calculated. All the results obtained are in complete agreement with computer simulation.