(1) f(x) = x+o(x) . The problem is discussed explicitly by Landau ([7], pp . 597-604 ; [8]), Ingham ([3]), Karamata ([5], [6]), Gordon ([1]), and is implicit in the 'Eratosthenian' summation method introduced by Wintner ([10], [11]) . In this paper we consider the analogous problem in which the sequence {n} of all positive integers is replaced by a finite or infinite sequence 1, a,, a2, . . . o...