---
title: "Euler's Partition Function Identity | Combinatorics"
description: "Euler's Partition Function Identity says partitions into distinct parts match partitions into odd parts, a core counting result in Combinatorics."
canonical: "https://fiveable.me/combinatorics/key-terms/eulers-partition-function-identity"
type: "key-term"
subject: "Combinatorics"
unit: "Unit 8"
---

# Euler's Partition Function Identity | Combinatorics

## Definition

Euler's Partition Function Identity says the number of partitions of n into distinct parts equals the number of partitions of n into odd parts. In Combinatorics, it shows up as a generating function identity.

## What It Is

Euler's Partition Function Identity is the combinatorics result that counts two very different kinds of partitions in exactly the same way: partitions into distinct parts and partitions into odd parts. So if you let d(n) be the number of partitions of n with no repeated part, and o(n) be the number of partitions of n using only odd parts, then d(n) = o(n).

This is not saying every partition is the same thing written two ways. It says the two counting problems produce the same number. For example, for n = 5, the distinct-part partitions are 5 and 4 + 1, while the odd-part partitions are 5 and 3 + 1 + 1. Both lists have 3 partitions, so d(5) = o(5) = 3.

The cleanest way to see the identity is with generating functions. Partitions into distinct parts have generating function (1 + x)(1 + x^2)(1 + x^3) ..., because each part size can be used either once or not at all. Partitions into odd parts have generating function 1/(1 - x)(1 - x^3)(1 - x^5) ..., because each odd part can be used any number of times. Euler showed these two products are equal after algebraic manipulation, so they encode the same coefficients.

There is also a combinatorial proof, which is often the most satisfying version in a class. You take a partition into distinct parts and break each part into powers of 2 times an odd number. Then the powers of 2 tell you how many copies to make, and the odd number tells you which odd part appears. That creates a bijection between the two kinds of partitions.

A common mistake is mixing this up with the ordinary partition function p(n). Euler's identity does not say the total number of partitions equals the number of distinct-part partitions. It says one restricted partition count equals another restricted partition count, which is much more specific and much more useful in generating function problems.

## Why It Matters

Euler's Partition Function Identity shows how combinatorics often compares two counting problems that look unrelated at first. That habit, finding a bijection or matching generating functions, shows up all over integer partitions, recurrences, and partition identities.

In a Combinatorics unit, this identity gives you a model for how proof by counting works. You are not just calculating a number, you are showing that two structures have the same size. That matters when a problem asks you to prove an equality between partition counts, simplify a generating function, or recognize a pattern in coefficients.

It also gives you a doorway into more advanced partition topics. Once you see why distinct-part and odd-part partitions match, later results like the Pentagonal Number Theorem or q-binomial theorem feel less mysterious because they live in the same generating-function world. The identity is one of those results that trains you to read product formulas as counting statements.

If your class uses recurrence relations or generating functions, this theorem is a great example of how a product expansion can encode a counting rule. It is a small identity with big payoff, because it connects algebraic manipulation to actual combinatorial objects.

## Connections

### Integer Partitions

Euler's identity only makes sense once you are comfortable with what a partition is and why order does not matter. The theorem compares two restricted types of partitions, so it sits right on top of the basic partition-counting idea. If you can list partitions of small numbers like 4, 5, or 6, you can test the identity by hand and see the pattern before moving to formulas.

### Generating Functions

The fastest algebraic proof of Euler's identity comes from generating functions. Distinct parts give factors of the form (1 + x^k), while odd parts give factors of the form 1/(1 - x^k) for odd k. The identity says these two infinite products are equal, which turns a counting statement into a product identity.

### [Conjugate Partition](/combinatorics/key-terms/conjugate-partition)

Conjugate partitions give a visual way to think about partition structure, especially when you draw Ferrers diagrams. While Euler's identity is not the same as conjugation, both ideas let you transform one partition viewpoint into another. In class, conjugate partitions often show up as a stepping stone toward seeing why different partition restrictions can still line up.

### [Pentagonal Number Theorem](/combinatorics/key-terms/pentagonal-number-theorem)

The Pentagonal Number Theorem is another famous Euler result about partition generating functions. If Euler's Partition Function Identity shows you how product formulas can count partitions, the Pentagonal Number Theorem shows that those same products can have surprisingly sparse coefficient patterns. The two results usually appear together in partition theory because they come from Euler's broader generating-function work.

## On the AP Exam

A problem set question might ask you to verify the identity for a small value of n, write the generating function for partitions into distinct parts, or explain why the odd-part and distinct-part counts match. The move is usually to list the partitions or set up the product formula, then compare coefficients. If the class wants a proof, you may be asked to describe the bijection idea in words instead of carrying out full algebra.

When you see a partition identity on a quiz, check whether the question is asking for a count, a generating function, or a combinatorial explanation. A quick example for n = 5 or n = 6 can often earn partial credit because it shows you know what each side is counting. For proof-based questions, name the restriction on parts carefully, since mixing up distinct, odd, and unrestricted partitions is the most common error.

## Euler's Partition Function Identity vs Partition Function

Euler's Partition Function Identity is a specific theorem about two restricted partition counts being equal. The partition function p(n) is the broader function that counts all partitions of n with no restriction on the parts. If you blur them together, you end up claiming the identity compares total partitions to distinct or odd partitions, which is not what Euler proved.

## Key Takeaways

- Euler's Partition Function Identity says partitions into distinct parts and partitions into odd parts are counted by the same number.
- This identity is usually written as d(n) = o(n), not as a statement about the full partition function p(n).
- The generating function for distinct parts uses factors like (1 + x^k), while the odd-part version uses factors like 1/(1 - x^k) for odd k.
- A bijective proof works by turning the structure of a distinct-part partition into an odd-part partition without changing the total.
- The identity is a standard example of how Combinatorics connects counting objects to algebraic product formulas.

## FAQs

### What is Euler's Partition Function Identity in Combinatorics?

It is the theorem that the number of partitions of n into distinct parts equals the number of partitions of n into odd parts. The identity is often shown with generating functions or a bijection. It is one of the classic results in partition theory.

### Is Euler's Partition Function Identity the same as the partition function p(n)?

No. The partition function p(n) counts all partitions of n, with repeats allowed and no restriction on parity. Euler's identity compares two restricted counts, distinct parts versus odd parts. That distinction matters a lot on homework and proofs.

### How do you prove Euler's Partition Function Identity?

The two most common proofs use generating functions or a bijection. With generating functions, you show the product for distinct parts equals the product for odd parts. With a bijection, you transform each distinct-part partition into an odd-part partition by tracking powers of 2.

### Can you give a small example of Euler's Partition Function Identity?

For n = 5, the distinct-part partitions are 5, 4 + 1, and 3 + 2. The odd-part partitions are 5, 3 + 1 + 1, and 1 + 1 + 1 + 1 + 1. Both sides give 3 partitions, which is exactly the identity in action.

## Related Study Guides

- [8.1 Integer partitions and partition functions](/combinatorics/unit-8/integer-partitions-partition-functions/study-guide/7bOCCKMIxT4Jm4K0)

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