Actual problem is
How many spaces $5$planes divide a space into?
and by some analogy and proof, I found that $5$planes divides a space into $26$spaces.
in fact, I considered first "How many spaces $4$planes divide a space into, and I found that there exists $15$ spaces and ONLY ONE of them is a CLOSED SPACE, not a infinite space(I don't know the real term).
SO I wonder
how many closed spaces in $26$spaces that $5$planes make?
$n$ points divide a line into $a_{0,n}=n+1$ intervals. $b_{0,n}=\max(n-1,0)$ of them are bounded.
Denote by $a_{1,n}$ the number of regions, that $n$ lines divide a plane and by $b_{1,n}$ the number of bounded regions. In $3$-dimensional space, let $a_{2,n}$ (or $b_{2,n}$) be the number of regions (or bounded regions), that $n$ planes divide a space.
You can calculate $b_{2,5}$ by recursion: $$ \begin{cases} b_{d,0}=0 & d\in\mathbb{N}\\ b_{0,n}=n-1 & n\in\mathbb{N}_+\\ b_{d+1,n+1}=b_{d+1,n}+b_{d,n}&d,n\in\mathbb{N}. \end{cases} $$