1.3. Simple estimation techniques 7 1.3. Simple estimation techniques In analytic number theory arithmetical questions are characteristically an- swered by finding estimates that imply the desired conclusions. The proof of Bertrand’s Postulate is typical we wanted to know that every interval (x/2,x] for x 2 contains at least one prime, but to obtain this result we detoured through a lower bound for ϑ(x) ϑ(x/2). Carrying out an esti- mate often requires long calculations with inequalities, for which the usual notation proves cumbersome. The big-O notation of P. G. H. Bachmann is well adapted for efficient calculations with long chains of inequalities. The statement f(x) = O(g(x)) means that there exist some unspecified con- stants C 0 and x0 such that |f(x)| Cg(x) on the interval x x0. The bound ψ(x) 2 log(2)x + log2(x)/ log(2) for x 1 would be expressed as ψ(x) = O(x) in the big-O notation. The latter statement is less precise, but the kind of information suppressed in the big-O notation is often unim- portant anyway. The following result is the key to labor-saving calculations using the big-O notation. Proposition 1.3. If f1(x) = O(g(x)) and f2(x) = O(g(x)) then f1(x) + f2(x) = O(g(x)). If also f3(x) = O(h(x)) then f1(x)f3(x) = O(g(x)h(x)). Proof. There are C1,C2,C3 0 and x1,x2,x3 such that |f1(x)| C1g(x) for x x1, |f2(x)| C2g(x) for x x2 and |f3(x)| C3h(x) for x x3. Then |f1(x) + f2(x)| |f1(x)| + |f2(x)| C1g(x) + C2g(x) = (C1 + C2)g(x) for x max(x1,x2) and |f1(x)f3(x)| = |f1(x)||f2(x)| C1g(x)C3h(x) = (C1C3)g(x)h(x) for x max(x1,x3). Note in particular that the largest of the O-terms in a sum absorbs everything smaller. The notation f(x) g(x) due to I. M. Vinogradov is synonymous with f(x) = O(g(x)). The notation f(x) g(x) of G. H. Hardy means that f(x) g(x) and g(x) f(x). There is also a small-o notation due to Landau. The statement f(x) = o(g(x)) means that limx→+∞ f(x)/g(x) = 0, or equivalently, for any ε 0 there exists some x0(ε) so that |f(x)| εg(x) for x x0(ε). If the function f in an estimate f = O(g) depends on one or more parameters, say fa(x) = Oa(g(x)), the question of uniformity of the estimate is often of considerable importance. If it is possible to choose both C and x0 in the statement that |fa(x)| Cg(x) for x x0
Previous Page Next Page