How do I read this triple summation? $\sum_{1\leq i < j < k \leq 4}a_{ijk}$ How do I read this triple summation? 
$$\sum_{1\leq i <  j < k \leq 4}a_{ijk}$$
The exercise asks me to express it as three sumations and to expand them in the following way:
1) Summing first on $k$, then on $j$ and last on $i$.
2) Summing first on $i$, then on $j$ and last on $k$.
My attempt:
$$
\sum_{k=3}^4\sum_{j=2}^{k-1}\sum_{i=1}^{j-1} a_{ijk}=\sum_{j=2}^2\sum_{i=1}^{j-1}a_{ij3}+\sum_{j=2}^3\sum_{i=1}^{j-1}a_{ij4}=\sum_{i=1}^1a_{i23}+\sum_{i=1}^{1}a_{i24}+\sum_{i=1}^2a_{i34}
\\=a_{123}+a_{124}+a_{134}+a_{234}$$
Is this correct so far? I don't know how to do the following parts of the exercise.
Could someone give me a general explanation on how to read this type of multiple summations?
Thanks all in advice.

When there is just one part of the sum, you should think that you're summing over a set. You should think of it as
$$\sum_{(i,j,k)\in S}a_{ijk}$$
where $S=\{(i,j,k)\mid1\le i<j<k\le4\}$
