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Suppose I have a partition $p$ of a positive integer $n$, $p$ is defined to be a friendly partition if and only if the following hold:

  1. $n$ is divisible by $4$.
  2. The length of $p$ is $\frac{n}{2}$.
  3. Every part in $n$ is less than or equal to $\frac{n}{2}$.

Let's define a partition $p$ to be splittable if and only if there are two partitions $p'$ and $p''$ such that $p$ is the same partition as $p' + p''$ and the sum of $p'$ and $p''$ are the same.

Empirically, it seems to be the case that every friendly partition is balanced, although I don't have a proof.

I'm curious whether a stronger condition holds. It seems to be possible in every case to split a friendly partition $p$ into two partitions, one of which is composed solely of $\{1, 2, 3, 4, \max(p)\}$ in some combination.

I'm curious whether it is always possible to split a friendly partition such that one of its parts is composed solely of $\{1, 2, 3, 4, \max(p)\}$.

As an example, the friendly partition $1+1+1+1+1+2+3+3+3+4$ can be split where one of its parititons is $3+3+4$, which satisfies the rule above.

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  • $\begingroup$ Sorry, I meant to rule that out. In my own notes I’ve sometimes used $n$ as the length of $p$ and sometimes as the sum. The length must be even. The sum must be divisible by $4$. $\endgroup$ Commented Dec 1, 2022 at 0:36

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For a counterexample, let $n=68$ and let $p$ be the partition of $68$ given by $$ 68=(1+\cdots+1)+(5+\cdots+5)+7 $$ where $1$ has multiplicity $26$, and $5$ has multiplicity $7$.

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  • $\begingroup$ That’s awesome! How did you find this counterexample? $\endgroup$ Commented Dec 1, 2022 at 4:36
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    $\begingroup$ @Greg Nisbet: Intuition as to a simple form for a likely counterexample, followed by some elementary algebra to find qualifying multiplicities. $\endgroup$
    – quasi
    Commented Dec 1, 2022 at 12:38

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