^^Baricentro   3+1   7+1   15+1.

 

Centro 3 1. Invariante: 3+1 = 4

X
X
X   O
-----
X X
X O
-----
 X
 X
 X
 O
-----
 
X
X
X       O
---------
X   X
X   O
---------
  X
  X
  X
  O
---------
 
X
X
X           O
-------------
X     X
X     O
-------------
   X
   X
   X
   O
-------------

X
X
X               O
-----------------
X       X
X       O
-----------------
    X
    X
    X
    O
-----------------

 

Centro 7 1. Invariante: 7+1 = 8

X
X
X
X
X
X
X       O
---------
X
X
X
X
X   X
X   O
---------
X X
X X
X X
X O
---------
 X
 X
 X
 X
 X
 X
 X
 O
---------
 
X
X
X
X
X
X
X               O
-----------------
X
X
X
X
X       X
X       O
-----------------
X   X
X   X
X   X
X   O
-----------------
  X
  X
  X
  X
  X
  X
  X
  O
-----------------

 

Centro 15 1. 15+1 = 16