1 Introduction
Let
$\mathbb {Z}$
be the set of all integers. For
$C,W\subseteq \mathbb {Z}$
and
$c\in \mathbb {Z}$
, define
For the set
$W=\{w\}$
consisting of a single element, we just write
$C+\{w\}$
as
$C+w$
for simplicity. If
$C+W=\mathbb {Z}$
, then C is called an additive complement to W. Furthermore, if no proper subset of C is an additive complement to W, then C is called a minimal additive complement to W. Sometimes, for simplicity, a minimal additive complement is just called a minimal complement throughout this article.
In 2011, Nathanson [Reference Nathanson9, Theorem 8] proved the following interesting theorem.
Theorem 1.1 (Nathanson’s theorem).
Let W be a nonempty, finite set of integers. In
$\mathbb {Z}$
, every complement to W contains a minimal complement to W.
Nathanson [Reference Nathanson9, Problem 11] also posed the following open problem.
Problem 1.2. Let W be an infinite set of integers. Does there exist a minimal complement to W? Does there exist a complement to W that does not contain a minimal complement?
In 2012, Chen and Yang [Reference Chen and Yang5, Theorem 1] considered Problem 1.2. For the situation
$\inf W=-\infty $
and
$\sup W=+\infty $
, they made the following progress.
Theorem 1.3 (Chen–Yang).
Let W be a set of integers with
$\inf W=-\infty $
and
$\sup W=+\infty $
. Then, there exists a minimal complement to W.
After Theorem 1.3, it therefore remains to deal with the case
$\inf W>-\infty $
or
$\sup W<+\infty $
. Without loss of generality, we may assume that
$\inf W>-\infty $
since, otherwise, we could consider
$-W$
instead of W. If
$\inf W=w_{0}>-\infty $
, then we can consider the set
instead of W. Thus, without loss generality, we may further assume that
$\inf W=1$
. Based on these observations, Chen and Yang [Reference Chen and Yang5, Theorem 2] proved the following theorem.
Theorem 1.4 (Chen–Yang).
Let
$W=\{1=w_1<w_2<\cdots \}$
be a set of integers and
where
$\mathbb {Z}^{+}$
is the set of all positive integers.
-
(a) If $\limsup _{i\rightarrow +\infty }(w_{i+1}-w_i)=+\infty $
, then there exists a minimal complement to W. -
(b) If $\lim _{i\rightarrow +\infty }(\overline {w_{i+1}}-\overline {w_i})=+\infty $
, then there does not exist a minimal complement to W.
In 2019, Kiss et al. [Reference Kiss, Sándor and Yang7] pointed out that if
then it is clear that
Hence, the criterion
$\lim _{i\rightarrow +\infty }(\overline {w_{i+1}}-\overline {w_i})=+\infty $
in Theorem 1.4(b) cannot be simply improved to
$\limsup _{i\rightarrow +\infty }(\overline {w_{i+1}}-\overline {w_i})=+\infty $
since this will lead to a contradiction between Theorems 1.4(a) and 1.4(b) for this W. Therefore, it is meaningful to think of the following question asked by Yang (private communication).
Problem 1.5. Let
$W=\{1=w_1<w_2<\cdots \}$
be a set of integers and
Determine the structure of sets W with
$\limsup _{i\rightarrow +\infty }(\overline {w_{i+1}}-\overline {w_i})=+\infty $
which do not have a minimal complement.
Along the same lines, Kiss et al. [Reference Kiss, Sándor and Yang7] investigated certain sets W with
which have a minimal complement. Precisely, let X be a set of integers. If there exists a positive integer T such that
$x+T\in X$
for all sufficiently large integers
$x\in X$
, then X is said to be eventually periodic with period T. If W is eventually periodic with
$|\mathbb {N}\setminus W|=+\infty $
, then there is some integer
$T_W$
so that
$w+T_W\in W$
for sufficiently large
$w\in W$
, which means that
Kiss, Sándor and Yang focused on eventually periodic sets and provided sufficient conditions and necessary conditions for eventually periodic sets to have a minimal additive complement.
For other related results about minimal additive complements, one may also refer to [Reference Alon, Kravitz and Larson1–Reference Chen and Fang4, Reference Davenport6–Reference Kwon8, Reference Ruzsa10–Reference Zhou12].
Extending Theorem 1.4(b), we answer Problem 1.5 by the following theorem which can be viewed as a partial solution to Problem 1.2.
Theorem 1.6. Let
$W=\{1=w_1<w_2<\cdots \}$
be a set of integers and
If there exists a sequence
$i_1<i_2<\cdots $
such that
then there does not exist a minimal complement to W.
The following example shows that our new theorem has some advantages compared with Theorem 1.4.
Example 1.7. Let
It is plain that
Clearly,
Thus, both Theorems 1.4(a) and 1.4(b) fail to determine whether such a W has a minimal additive complement. However, it is easy to see that
and
Therefore, W does not have a minimal additive complement, thanks to the criterion from Theorem 1.6 with
$\overline {w_{i_{k}}}=2^k+8$
and
$\overline {w_{i_{k+1}}}=2^{k+1}+8$
.
Remark 1.8. We now give a simple explanation to show that the criterion given in Theorem 1.6 is essentially optimal. We first note that
for those W assumed in Theorem 1.6 which are consistent with the requirement of Problem 1.5. We next show that the condition
cannot be neglected. To see this, let
$W=\bigcup _{k=0}^{+\infty } [10^k,2\times 10^k]$
. It is clear that for this W, there exists a sequence
$i_1<i_2<\cdots $
with
$\overline {w_{i_t}}=10^{t+1}-1$
such that
Then, there exists a minimal additive complement to W by Theorem 1.4(a) since
$\limsup _{i\rightarrow +\infty }(w_{i+1}-w_i)=+\infty $
.
2 Proof of Theorem 1.6
The proof of Theorem 1.6 follows from that of Theorem 1.4(b) with refinements.
Suppose to the contrary that C is a minimal additive complement to W. Then, we clearly have
$\inf C=-\infty $
. Let
For any
$s\in \mathbb {Z}^+$
, we assume that
Since
$\limsup _{t\rightarrow +\infty }|\overline {W}\cap (\overline {w_{i_{t}}},\overline {w_{i_{t+1}}})|<+\infty $
, there is a constant K such that
which means that there is some
$t_W$
such that for any
$t\ge t_W$
,
Our proof will be separated into three cases.
Case 1:
$\liminf _{s\rightarrow +\infty } (-c_s-\overline {w_{i_{u_s-1}+1}})<+\infty $
. Let
Then,
$0\le h<+\infty $
. Define
It is clear that H is an infinite set since
$-c_s-\overline {w_{i_{u_s-1}+1}}$
are all integers. Let
$s_0\in H$
be any given integer. Then, there exist integers
$n_{s_0}\in \mathbb {Z}$
and
$w_{s_0}\in W$
such that
for any
$w\in W$
and
$c\in C$
with
$c\neq c_{s_0}$
, for otherwise,
$C\setminus \{c_{s_0}\}$
will be an additive complement to W, which contradicts the minimal property of C. Recall that
${\inf C=-\infty }$
and
from the condition of our theorem. There exists an integer
$s_0^*> s_0$
such that
for all
$s\in H$
with
$s\ge s_0^*$
and
for all
$t\geq u_{s_0^*}-1$
. From (2.3) and (2.4),
for
$s\ge s_0^*$
. From (2.5) with
$t=u_s$
,
providing that
$s\ge s_0^*$
. In other words,
However, again from (2.5) with
$t=u_s-1$
,
provided that
$s\ge s_0^*$
. We conclude from (2.6), (2.7) and (2.9) that
for all
$s_0,s\in H$
with
$s\ge s_0^*> s_0$
.
Now, we take
$K+2$
different integers
$s_{0,1},s_{0,2},\ldots ,s_{0,K+2}\in H$
. Then, there are
$K+2$
integers
$s_{0,1}^*,s_{0,2}^*,\ldots ,s_{0,K+2}^*$
such that for any j with
$1\le j\le K+2$
,
from (2.10). Now, let s be a sufficiently large integer so that
and
$u_s-1\ge t_W$
. Then, for such s,
from (2.11). Moreover, from (2.2),
from which it follows that there are at least two integers
$j_1, j_2$
with
$1\le j_1<j_2\le K+2$
such that
By (2.3), we clearly have
which is a contradiction with (2.3) since
$n_{s_{0,{j_1}}}=n_{s_{0,{j_2}}}$
whereas
$c_{s_{0,{j_1}}}\neq c_{s_{0,{j_2}}}$
.
Case 2:
$\liminf _{s\rightarrow +\infty } (c_s+\overline {w_{i_{u_s}+1}})<+\infty $
. The proof is similar to Case 1. Let
Then,
$0\le \ell <+\infty $
. Define
It is clear that L is an infinite set since
$c_s+\overline {w_{i_{u_s}+1}}$
are all integers. Let
$s_0\in H$
be any given integer. Then, there exist integers
$n_{s_0}\in \mathbb {Z}$
and
$w_{s_0}\in W$
such that
for any
$w\in W$
and
$c\in C$
with
$c\neq c_{s_0}$
, for otherwise,
$C\setminus \{c_{s_0}\}$
will be an additive complement to W, which contradicts with the minimal property of C. Since
${\inf C=-\infty }$
and
from the condition of our theorem, there exists an integer
$s_0^*> s_0$
such that
for all
$s\in L$
with
$s\ge s_0^*$
and
for all
$t\geq u_{s_0^*}$
. From (2.12) and (2.13),
for
$s\ge s_0^*$
. By (2.14) with
$t=u_s+1$
,
providing that
$s\ge s_0^*$
. From (2.1) and (2.16) for
$s\ge s_0^*$
,
However, using (2.16) again,
by the definition of
$\ell $
, from which it follows that
provided that
$s\ge s_0^*$
. We conclude from (2.15), (2.17) and (2.18) that
for all
$s_0,s\in L$
with
$s\ge s_0^*> s_0$
.
The remaining arguments are the same as in the last paragraph of Case 1, so we omit them.
Case 3:
$ \liminf _{s\rightarrow +\infty } (-c_s-\overline {w_{i_{u_s-1}+1}})= \liminf _{s\rightarrow +\infty } (c_s+\overline {w_{i_{u_s}+1}})=+\infty $
. Since C is a minimal complement to W, for any given integer
$s_0\geq 1$
, there exist integers
$n_{s_0}\in \mathbb {Z}$
and
$w_{s_0}\in W$
such that
for any
$w\in W$
and
$c\in C$
with
$c\neq c_{s_0}$
. By the conditions of Case 3, there exist infinitely many integers
$s>s_0$
such that
which implies that
We conclude from (2.19) and (2.20) that
for any
$s>s_0$
.
Using similar arguments to those in Case 1, the validity of the last case clearly follows.
