In number theory, a partition of a positive integer n is a way of writing n as a sum of positive integers. Two sums that differ only in the order of their summands are considered to be the same partition; if order matters then the sum becomes a composition. A summand in a partition is also called a part. The number of partitions of n is given by the partition function p(n).
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The partitions of 4 are listed below:
The partitions of 8 are listed below:
In number theory, the partition function p(n) represents the number of possible partitions of a natural number n, which is to say the number of distinct (and order independent) ways of representing n as a sum of natural numbers. For example, 4 can be partitioned in five distinct ways:
So p(4) = 5. By convention p(0) = 1, p(n) = 0 for n negative. Partitions can be graphically visualized with Young diagrams. They occur in a number of branches of mathematics and physics, including the study of symmetric polynomials, the symmetric group and in group representation theory in general.
One way of getting a handle on the partition function involves an intermediate function p(k, n), which represents the number of partitions of n using only natural numbers at least as large as k. For any given value of k, partitions counted by p(k, n) fit into exactly one of the following categories:
The number of partitions meeting the first condition is p(k, n − k). To see this, imagine a list of all the partitions of the number n − k into numbers of size at least k, then imagine appending "+ k" to each partition in the list. Now what is it a list of? As a side note, one can use this to define a sort of recursion relation for the partition function in term of the intermediate function, namely
where is the floor function.
The number of partitions meeting the second condition is p(k + 1, n) since a partition into parts of at least k that contains no parts of exactly k must have all parts at least k + 1.
Since the two conditions are mutually exclusive, the number of partitions meeting either condition is p(k + 1, n) + p(k, n − k). The recursively defined function is thus:
This function tends to exhibit deceptive behavior.
Our original function p(n) is just p(1, n).
The values of this function:
k  

1  2  3  4  5  6  7  8  9  10  
n  1  1  0  0  0  0  0  0  0  0  0 
2  2  1  0  0  0  0  0  0  0  0  
3  3  1  1  0  0  0  0  0  0  0  
4  5  2  1  1  0  0  0  0  0  0  
5  7  2  1  1  1  0  0  0  0  0  
6  11  4  2  1  1  1  0  0  0  0  
7  15  4  2  1  1  1  1  0  0  0  
8  22  7  3  2  1  1  1  1  0  0  
9  30  8  4  2  1  1  1  1  1  0  
10  42  12  5  3  2  1  1  1  1  1 
A generating function for p(n) is given by the reciprocal of Euler's function:
Expanding each term on the righthand side as a geometric series, we can rewrite it as
The x^{n} term in this product counts the number of ways to write
where each number i appears a_{i} times. This is precisely the definition of a partition of n, so our product is the desired generating function. More generally, the generating function for the partitions of n into numbers from a set A can be found by taking only those terms in the product where k is an element of A. This result is due to Euler.
The formulation of Euler's generating function is a special case of a qPochhammer symbol and is similar to the product formulation of many modular forms, and specifically the Dedekind eta function. It can also be used in conjunction with the pentagonal number theorem to derive a recurrence for the partition function stating that:
where the sum is taken over all generalized pentagonal numbers of the form ½n(3n − 1), for n running over positive and negative integers: successively taking n = 1, −1, 2, −2, 3, −3, 4, −4, ..., generates the values 1, 2, 5, 7, 12, 15, 22, 26, 35, 40, 51, .... The signs in the summation continue to alternate +, +, −, −, +, +, ...
Some values of the partition function are as follows (sequence A000041 in OEIS):
As of February 2010, the largest known prime number of this kind is p(29099391), with 6002 decimal digits.^{[1]}
An asymptotic expression for p(n) is given by
This expression was first obtained by G. H. Hardy and Ramanujan in 1918 and independently by J. V. Uspensky in 1920. Considering p(1000), the asymptotic formula gives about 2.4402 × 10^{31}, reasonably close to the exact answer given above.
In 1937, Hans Rademacher was able to improve on Hardy and Ramanujan's results by providing a convergent series expression for p(n). It is
where
Here, the notation (m, n) = 1 implies that the sum should occur only over the values of m that are relatively prime to n. The function s(m, k) is a Dedekind sum. The proof of Rademacher's formula is interesting in that it involves Ford circles, Farey sequences, modular symmetry and the Dedekind eta function in a central way.
Srinivasa Ramanujan is credited with discovering that "congruences" in the number of partitions exist for integers ending in 4 and 9.
For instance, the number of partitions for the integer 4 is 5. For the integer 9, the number of partitions is 30; for 14 there are 135 partitions. He also discovered congruences related to 7 and 11:
Since 5, 7, and 11 are consecutive primes, one might think that there would be such a congruence for the next prime 13, for some a. This is, however, false.
In the 1960s, A. O. L. Atkin of the University of Illinois at Chicago discovered additional congruences for small prime moduli. For example:
In 2000, Ken Ono of the University of Wisconsin–Madison proved that there are such congruences for every prime modulus. A few years later Ono, together with Scott Ahlgren of the University of Illinois, proved that there are partition congruences modulo every integer coprime to 6.^{[2]}
Among the 22 partitions for the number 8, 6 contain only odd parts:
If we count the partitions of 8 with distinct parts, we also obtain the number 6:
It is true for all positive numbers that the number of partitions with odd parts always equals the number of partitions with distinct parts. This result was proved by Leonard Euler in 1748.^{[3]}
Some similar results about restricted partitions can be obtained by the aid of a visual tool, a Ferrers graph (also called Ferrers diagram, since it is not a graph in the graphtheoretical sense, or sometimes Young diagram, alluding to the Young tableau).
The partition 6 + 4 + 3 + 1 of the positive number 14 can be represented by the following diagram; these diagrams are named in honor of Norman Macleod Ferrers:
6+4+3+1 
The 14 circles are lined up in 4 columns, each having the size of a part of the partition. The diagrams for the 5 partitions of the number 4 are listed below:
4  =  3+1  =  2+2  =  2+1+1  =  1+1+1+1 
If we now flip the diagram of the partition 6 + 4 + 3 + 1 along its main diagonal, we obtain another partition of 14:
↔  
6+4+3+1  =  4+3+3+2+1+1 
By turning the rows into columns, we obtain the partition 4 + 3 + 3 + 2 + 1 + 1 of the number 14. Such partitions are said to be conjugate of one another. In the case of the number 4, partitions 4 and 1 + 1 + 1 + 1 are conjugate pairs, and partitions 3 + 1 and 2 + 1 + 1 are conjugate of each other. Of particular interest is the partition 2 + 2, which has itself as conjugate. Such a partition is said to be selfconjugate.
Claim: The number of selfconjugate partitions is the same as the number of partitions with distinct odd parts.
Proof (sketch): The crucial observation is that every odd part can be "folded" in the middle to form a selfconjugate diagram:
↔ 
One can then obtain a bijection between the set of partitions with distinct odd parts and the set of selfconjugate partitions, as illustrated by the following example:
↔  
9+7+3  =  5+5+4+3+2 
dist. odd  selfconjugate 
Similar techniques can be employed to establish, for example, the following equalities:
An elementary introduction to the topic of integer partition, including a discussion of Ferrers graphs, can be found in the following reference:
Another highly accessible introduction is:
