In additive number theory, the Schnirelmann density of a sequence of numbers is a way to measure how "dense" the sequence is. It is named after Russian mathematician L.G. Schnirelmann, who was the first to study it.
The Schnirelmann density of a set of natural numbers A is defined as
The Schnirelmann density is well-defined even if the limit of A(n)/n as n → ∞ fails to exist (see asymptotic density).
By definition, 0 ≤ A(n) ≤ n and n σA ≤ A(n) for all n, and therefore 0 ≤ σA ≤ 1, and σA = 1 if and only if A = N. Furthermore,
The Schnirelmann density is sensitive to the first values of a set:
Consequently, the Schnirelmann densities of the even numbers and the odd numbers, which one might expect to agree, are 0 and 1/2 respectively. Schnirelmann and Yuri Linnik exploited this sensitivity as we shall see.
If we set , then Lagrange's four-square theorem can be restated as . (Here the symbol denotes the sumset of and .) It is clear that . In fact, we still have , and one might ask at what point the sumset attains Schnirelmann density 1 and how does it increase. It actually is the case that and one sees that sumsetting once again yields a more populous set, namely all of . Schnirelmann further succeeded in developing these ideas into the following theorems, aiming towards Additive Number Theory, and proving them to be a novel resource (if not greatly powerful) to attack important problems, such as Waring's problem and Goldbach's conjecture.
Theorem. Let and be subsets of . Then
Note that . Inductively, we have the following generalization.
Corollary. Let be a finite family of subsets of . Then
The theorem provides the first insights on how sumsets accumulate. It seems unfortunate that its conclusion stops short of showing being superadditive. Yet, Schnirelmann provided us with the following results, which sufficed for most of his purpose.
Theorem. Let and be subsets of . If , then
Theorem. (Schnirelmann) Let . If then there exists such that
A subset with the property that for a finite sum, is called an additive basis, and the least number of summands required is called the degree (sometimes order) of the basis. Thus, the last theorem states that any set with positive Schnirelmann density is an additive basis. In this terminology, the set of squares is an additive basis of degree 4.
Theorem. (Mann 1942) Let and be subsets of . In case that , we still have
An analogue of this theorem for lower asymptotic density was obtained by Kneser.
Let and be natural numbers. Let . Define to be the number of non-negative integral solutions to the equation
and to be the number of non-negative integral solutions to the inequality
in the variables , respectively. Thus . We have
The volume of the -dimensional body defined by , is bounded by the volume of the hypercube of size , hence . The hard part is to show that this bound still works on the average, i.e.,
Lemma. (Linnik) For all there exists and a constant , depending only on , such that for all ,
With this at hand, the following theorem can be elegantly proved.
Theorem. For all there exists for which .
We have thus established the general solution to Waring's Problem:
Corollary. (Hilbert 1909) For all there exists , depending only on , such that every positive integer can be expressed as the sum of at most many -th powers.
In 1930 Schnirelmann used these ideas in conjunction with the Brun sieve to prove Schnirelmann's theorem, that any natural number greater than one can be written as the sum of not more than C prime numbers, where C is an effectively computable constant: Schnirelmann obtained C < 800000. Schnirelmann's constant is the lowest number C with this property.
Khintchin proved that the sequence of squares, though of zero Schnirelmann density, when added to a sequence of Schnirelmann density between 0 and 1, increases the density:
This was soon simplified and extended by Erdős, who showed, that if A is any sequence with Schnirelmann density α and B is an additive basis of order k then
and this was improved by Plünnecke to
Sequences with this property, of increasing density less than one by addition, were named essential components by Khintchin. Linnik showed that an essential component need not be an additive basis as he constructed an essential component that has xo(1) elements less than x. More precisely, the sequence has
elements less than x for some c < 1. This was improved by E. Wirsing to
For a while, it remained an open problem how many elements an essential component must have. Finally, Ruzsa determined that an essential component has at least (log x)c elements up to x, for some c > 1, and for every c > 1 there is an essential component which has at most (log x)c elements up to x.
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