We study the restrictions, the strict fixed points, and the strict quotients of the partition complex $|\Pi_{n}|$
|
Π
n
|
, which is the $\Sigma_{n}$
Σ
n
-space attached to the poset of proper nontrivial partitions of the set $\{1,\ldots,n\}$
{
1
,
…
,
n
}
.We express the space of fixed points $|\Pi_{n}|^{G}$
|
Π
n
|
G
in terms of subgroup posets for general $G\subset \Sigma_{n}$
G
⊂
Σ
n
and prove a formula for the restriction of $|\Pi_{n}|$
|
Π
n
|
to Young subgroups $\Sigma_{n_{1}}\times \cdots\times \Sigma_{n_{k}}$
Σ
n
1
×
⋯
×
Σ
n
k
. Both results follow by applying a general method, proven with discrete Morse theory, for producing equivariant branching rules on lattices with group actions.We uncover surprising links between strict Young quotients of $|\Pi_{n}|$
|
Π
n
|
, commutative monoid spaces, and the cotangent fibre in derived algebraic geometry. These connections allow us to construct a cofibre sequence relating various strict quotients $|\Pi_{n}|^{\diamond} \mathbin {\operatorname* {\wedge }_{\Sigma_{n}}^{}} (S^{\ell})^{\wedge n}$
|
Π
n
|
⋄
∧
Σ
n
(
S
ℓ
)
∧
n
and give a combinatorial proof of a splitting in derived algebraic geometry.Combining all our results, we decompose strict Young quotients of $|\Pi_{n}|$
|
Π
n
|
in terms of “atoms” $|\Pi_{d}|^{\diamond} \mathbin {\operatorname* {\wedge }_{\Sigma_{d}}^{}} (S^{\ell})^{\wedge d}$
|
Π
d
|
⋄
∧
Σ
d
(
S
ℓ
)
∧
d
for $\ell$
ℓ
odd and compute their homology. We thereby also generalise Goerss’ computation of the algebraic André-Quillen homology of trivial square-zero extensions from $\mathbf {F}_{2}$
F
2
to $\mathbf {F}_{p}$
F
p
for $p$
p
an odd prime.