Many authors have studied strong laws of large numbers (SLLN) for arrays of random variables or, more generally, random fields with certain dependence structures. For example, Klesov [8, 9] proved a strong law of large numbers for orthogonal random fields and related asymptotic properties. Móricz [13, 14, 15, 16, 17, 18] established SLLN for quasi-orthogonal or quasi-stationary random fields. Móricz, Stadtmüller and Thalmaier  proved SLLN for blockwise -dependent random fields under moment conditions. Thanh  provided a necessary and sufficient condition for a general -dimensional arrays of random variables to satisfy a strong law of large numbers, which can be applied to blockwise independent random fields.
For random fields with more information on their dependence structures such as linear random fields, more results on their limiting behaviors have been known. For example, Marinucci and Poghosyan , and Paulauskas 
studied the asymptotics for linear random fields, including law of large numbers, central limit theorems and invariance principles, by applying the Beveridge-Nelson decomposition method. By applying ergodic theory, Banys et al. studied strong law of large numbers of linear random fields generated by ergodic or mixing random variables. Recently, Sang and Xiao  proved exact moderate and large deviation results for linear random fields with independent innovations and applied them to establish laws of the iterated logarithm for linear random fields.
This paper is mainly motivated by our interest in asymptotic properties of fractional random fields with long-range dependence and/or heavy-tailed distributions. An important class of such random fields is formed by fractional stable fields (cf. e.g., Samorodnitsky and Taqqu , Cohen and Istas , Pipiras and Taqqu , Ayache, Roueff and Xiao , Xiao ).
We start with some notation and definitions. Let be the set of non-negative integers. For any , we write it as or . The -norm and -norm of are defined by
respectively. For any constant , we denote .
There is a natural partial order in : if and only if for all . For any , we denote , which is called an interval (or a rectangle) in .
Let be a real-valued random field indexed by the lattice and let be a sequence of increasing subsets of . Denote the partial sum of over by
Let be a function such that as We say that a random field satisfies the strong law of large numbers with respect to and if
In the random field setting, the index sets can have various configurations, ranging from spherical to rectangular, and to non-standard shapes, which arise naturally in many applied areas such as spatial statistics. When is clear from the context, we will simply refer to (1.1) as a -SLLN.
In this paper, we will consider spherical and rectangular sets , see below. Móricz  considered SLLN over more types of increasing domains in .
Two types of functions are of particular interest in this paper:
, where is a non-decreasing function such that as . Such functions are useful in establishing SLLN for (approximately) isotropic random fields, including a class of stable random fields with stationary increments in .
Throughout this paper, is a constant. We first consider SLLN for partial sums over spherical domains , where for any ,
If we replace the -norm by the -norm , then is the cube .
Let . We denote
The following SLLN holds under both and -norms. It can be applied to isotropic random fields.
Let be a non-decreasing function such that as . Assume that there are constants such that for all . Let be an integer that satisfies . If
The second theorem proves a SLLN for partial sums over rectangular domains , that can be applied to anisotropic random fields.
For any , denote by
the partial sum of the random variables over the interval .
Suppose that, for , is an increasing function that satisfy for all for some constants . Assume is an integer that satisfies . If
then satisfies the SLLN (1.1) with and .
The following direct consequence of Theorem 1.2 is convenient to use.
The method for proving Theorem 1.2 is different from that based on the Rademacher-Menshov-type maximal moment inequalities (e.g., [8, 9, 13, 14, 15, 16, 17, 18, 19, 28]). We rely more on the approach of Chobanyan et al. [6, 11] and Nane et al. . In particular, we extend their maximal moment inequalities for sequences of random variables to the case of random fields. We should mention that it is more convenient to verify condition (1.6) when the random field has certain kind of stationarity (see Section 4).
The rest of this paper is organized as follows. In Section 2 and 3, we prove Theorems 1.1 and 1.2, respectively. Some applications of these theorems are given in Section 4, where various random fields including fractional stable random fields, orthogonal, and quasi-stationary random fields are considered.
2. Spherical sums: back to the one-dimensional case
In this section we prove Theorem 1.1.
Proof of Theorem 1.1.
Recall in (1.2), and let We divide the proof into two steps.
Step 1: Our first task is to establish a recursion for . We first consider the case . For any , notice that
and in general for ,
By using the elementary inequality we get
Eq. (2.1) can be written as
For , define
By the definition of , in both -norm and -norm. Hence .
Dividing both sides of (2.2) by , taking expectations, and then the supremum over all the ’s, we get
The terms in contribute nicely in the recursion for because of the following observation. By the assumption, we have
Consequently, for , we have almost surely
Note that (1.3) implies that
It follows from this and (2.7) that almost surely
3. Rectangular sums: the chaining method
Let be a set of non-negative numbers with the following two properties: There is a finite constant such that
for all . If is a sequence of real numbers such that
For , define
We call the -dimensional maximal sum of size , and the -dimensional maximal sum. We will prove several maximal moment inequalities. The first is for the case of .
Let and let () be the functions as in Theorem 1.2. There is a constant such that for ,
We start by establishing a recursion for in for each . By definition (3.4) we see that for ,
It follows from (3.7) and the elementary inequality that
By iterating the above inequality, we derive
By adding and subtracting terms in (3.8), we get
Dividing both sides by , taking expectations and then the supremum over we get,
The terms in help us get the recursion we need. By the assumption on , we have
Thus we get the recursion formula for :
Summing this recursion over , we get
With essentially the same argument as above we get for that
Thus, we obtain (3.6) with . ∎
The next lemma establishes a maximal moment inequality for .
Let and let () be as in Theorem 1.2. There exist constants and such that for ,
We start with establishing a recursion for in for each . Observe that for ,