In mathematics, solid partitions are natural generalizations of integer partitions and plane partitions defined by Percy Alexander MacMahon. A solid partition of n {\displaystyle n} is a three-dimensional array of non-negative integers n i , j , k {\displaystyle n_{i,j,k}} (with indices i , j , k ≥ 1 {\displaystyle i,j,k\geq 1} ) such that
∑ i , j , k n i , j , k = n {\displaystyle \sum _{i,j,k}n_{i,j,k}=n}
and
n i + 1 , j , k ≤ n i , j , k , n i , j + 1 , k ≤ n i , j , k and n i , j , k + 1 ≤ n i , j , k {\displaystyle n_{i+1,j,k}\leq n_{i,j,k},\quad n_{i,j+1,k}\leq n_{i,j,k}\quad {\text{and}}\quad n_{i,j,k+1}\leq n_{i,j,k}} for all i , j and k . {\displaystyle i,j{\text{ and }}k.}
Let p 3 ( n ) {\displaystyle p_{3}(n)} denote the number of solid partitions of n {\displaystyle n} . As the definition of solid partitions involves three-dimensional arrays of numbers, they are also called three-dimensional partitions in notation where plane partitions are two-dimensional partitions and partitions are one-dimensional partitions. Solid partitions and their higher-dimensional generalizations are discussed in the book by Andrews.
Ferrers diagrams for solid partitions Another representation for solid partitions is in the form of Ferrers diagrams. The Ferrers diagram of a solid partition of n {\displaystyle n} is a collection of n {\displaystyle n} points or nodes, λ = ( y 1 , y 2 , … , y n ) {\displaystyle \lambda =(\mathbf {y} _{1},\mathbf {y} _{2},\ldots ,\mathbf {y} _{n})} , with y i ∈ Z ≥ 0 4 {\displaystyle \mathbf {y} _{i}\in \mathbb {Z} _{\geq 0}^{4}} satisfying the condition:
Condition FD: If the node a = ( a 1 , a 2 , a 3 , a 4 ) ∈ λ {\displaystyle \mathbf {a} =(a_{1},a_{2},a_{3},a_{4})\in \lambda } , then so do all the nodes y = ( y 1 , y 2 , y 3 , y 4 ) {\displaystyle \mathbf {y} =(y_{1},y_{2},y_{3},y_{4})} with 0 ≤ y i ≤ a i {\displaystyle 0\leq y_{i}\leq a_{i}} for all i = 1 , 2 , 3 , 4 {\displaystyle i=1,2,3,4} . For instance, the Ferrers diagram
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