feat(OEIS): A063880

[rwst]

Resolves #1455.

Conjectures associated with A063880

A063880 lists numbers $n$ such that $\sigma(n) = 2 \cdot \text{usigma}(n)$, where $\sigma(n)$ is the
sum of all divisors and $\text{usigma}(n)$ is the sum of unitary divisors.

Equivalently, these are numbers whose unitary and non-unitary divisors have equal sum.

The conjectures state that all members satisfy $n \equiv 108 \pmod{216}$, and that all
primitive terms (those whose proper divisors aren't in the sequence) are powerful numbers,
with $108$ being the only primitive term.

References: oeis.org/A063880

Note: I'm using Claude + Opus for supervised formalization tasks. Claude has no permission to use git on my machine.