Fix links missing .md

Author: HactarCE

docs/puzzles/physical/index.md

@@ -6,7 +6,7 @@
 !!! info inline end "Physical 3×3×3×3"
     ![Grant's physical 3×3×3×3](https://assets.hypercubing.xyz/img/phys/grant_3x3x3x3_render.png){width="100%"}
 
-Physical puzzles refer to higher dimensional puzzles that are physically built in real life (using only 3 dimensions). This involves clever designs, strange symmetry-abusing tricks, and lots and lots of magnets. Often, these puzzles are too impractical to use, defeating their sole purpose. For an explanation of how these puzzles work, see [the theory page](/theory/physical).
+Physical puzzles refer to higher dimensional puzzles that are physically built in real life (using only 3 dimensions). This involves clever designs, strange symmetry-abusing tricks, and lots and lots of magnets. Often, these puzzles are too impractical to use, defeating their sole purpose. For an explanation of how these puzzles work, see [the theory page](/theory/physical.md).
 
 For a documentary about the history of physical puzzles as of December 2022, see [Rowan's video](https://www.youtube.com/watch?v=QTc-rG-nunA).
 

docs/theory/physical.md

@@ -15,7 +15,7 @@ Since in a virtual puzzle, stickers are part of pieces and cannot be separated,
 
 The 2^4^ has 16 pieces, each with 4 stickers. By performing moves on a virtual 2^4^, a piece can be brought back to its position with any even permutation of stickers. (It is not just the [monoflip](/theory/invariants.md#monoflip) because that assumes the rest of the puzzle is solved, which is not important here.) In three dimensions, a tetrahedron with its four vertices representing the stickers has the same symmetry. (This is not a coincidence; a 2^4^ built on the surface of a hypersphere will have its pieces be spherical tetrahedra with the stickers at the vertices.) Thus, the pieces of Melinda's 2^4^ have tetrahedral symmetry.
 
-This is not the only design decision resulting in a usable physical puzzle. Since the pieces of the 2^4^ can be arranged in two cubic layers, the pieces are cubes to allow them to stick together in this fashion. This makes many of the moves simply rotating one whole cube of 8 pieces, and the others rotate 4 pieces from each layer in a simple way. These moves are unable to swap a white or yellow sticker with a sticker of a different color, which means the puzzle simulates a cubic prism puzzle: a 2^3^×2^1^ cuboid, where the first three axes can turn into each other but the last one cannot in the same way that a [3×3×3×2](/puzzles/physical/2x3x3x3) is restricted.
+This is not the only design decision resulting in a usable physical puzzle. Since the pieces of the 2^4^ can be arranged in two cubic layers, the pieces are cubes to allow them to stick together in this fashion. This makes many of the moves simply rotating one whole cube of 8 pieces, and the others rotate 4 pieces from each layer in a simple way. These moves are unable to swap a white or yellow sticker with a sticker of a different color, which means the puzzle simulates a cubic prism puzzle: a 2^3^×2^1^ cuboid, where the first three axes can turn into each other but the last one cannot in the same way that a [3×3×3×2](/puzzles/physical/2x3x3x3.md) is restricted.
 
 The virtual 2^4^, on the other hand, does allow moves that take a white or yellow sticker to a sticker of a different color, and the pieces were designed so that this is possible. It would be possible to observe each move that does that, record which piece goes where, and apply that to the physical 2^4^, but this is unwieldy. Instead, Melinda introduced a *gyro*, a sequence of actions that does the equivalent of a whole-puzzle rotation on the virtual puzzle, but takes a white sticker to a sticker on a different axis. This means other moves are accessible by performing the gyro and then a simple move. On this puzzle, a gyro is usually about six physical twisting actions, but on other puzzles, they can be more complicated, so finding a puzzle that supports a short gyro is valuable.
 
@@ -57,4 +57,4 @@ This does not preclude other designs for a physical 2^6 that have not been devel
 There are several ways to restrict a 2^6^ to make it able to be made physical. One way is to make it a 2^5^×2^1^, which can be made physical out of two copies of the 2^5^ in the same way as the 2^4^×2^1^ was made out of the 2^4^. Another way is the 2^4^×2^2^, pictured above. Each piece of the 2^4^×2^2^ has four 4-stickers and two 2-stickers. The four 4-stickers can have an even permutation applied to them, or the two 2-stickers can be swapped and an odd permutation can be applied to the four 4-stickers. This enlarges the symmetry group of the piece from $A_4$ to $S_4$, the symmetric group on 4 points. Fortunately, octahedral symmetry acts on the four space diagonals of a cube by $S_4$, and one endpoint from each diagonal corresponds to the vertices of a tetrahedron, used for the stickers on the 2^4^. Therefore, the 4-stickers can be placed on opposite corners of a cube. The stickers on the periphery of the pieces correspond to the 2-stickers, which swap whenever the piece is rotated in a way that does not preserve the original tetrahedron, i.e. one that does an odd permutation. It may require one or more buffer layers.
 
 
-[^1]: Images created with [Stella software](http://www.software3d.com/Stella.php).
+[^1]: Images created with [Stella software](http://www.software3d.com/Stella.php).