# Some useful algebra results

Starting with the original 9 cube equations, for some purposes it is convenient to expand them into the following 12 equations.

bc - ad = A1 ce - ag = A2 be - af = A3 cf - ah = (B1 + B2)/2 bg - de = (B1 - B2)/2 bg - ah = (B1 + B3)/2 cf - de = (B1 - B3)/2 de - ah = (B2 + B3)/2 cf - bg = (B2 - B3)/2 fg - eh = C1 df - bh = C2 dg - ch = C3

## Equations linear in the cube values

The above 12 equations can then be manipulated to form relationships that are linear in the cube values. As they are derived from the cube equations, they must hold for any cube solution.

a C3 + g A1 = c (B1 + B3)/2 a C1 + g A3 = e (B1 + B3)/2 b C3 + h A1 = d (B1 + B3)/2 b C1 + h A3 = f (B1 + B3)/2 e A1 - c A3 = a (B1 - B3)/2 f A1 - d A3 = b (B1 - B3)/2 c C1 - e C3 = g (B1 - B3)/2 d C1 - f C3 = h (B1 - B3)/2 e A1 - b A2 = a (B1 - B2)/2 g A1 - d A2 = c (B1 - B2)/2 b C1 - e C2 = f (B1 - B2)/2 d C1 - g C2 = h (B1 - B2)/2 a C2 + f A1 = b (B1 + B2)/2 c C2 + h A1 = d (B1 + B2)/2 a C1 + f A2 = e (B1 + B2)/2 c C1 + h A2 = g (B1 + B2)/2 b A2 - c A3 = a (B2 - B3)/2 f A2 - g A3 = e (B2 - B3)/2 c C2 - b C3 = d (B2 - B3)/2 g C2 - f C3 = h (B2 - B3)/2 d A3 + a C2 = b (B3 + B2)/2 a C3 + d A2 = c (B3 + B2)/2 h A3 + e C2 = f (B3 + B2)/2 e C3 + h A2 = g (B3 + B2)/2

If given values for the tuples (A1,B1,C1), (A2,B2,C2), (A3,B3,C3), this is now a system of linear equations for the cube values. Of the 24 linear equations, only 6 are linearly independent, so the 8 cube values can be solved with 2 freedoms remaining.

These freedoms are just from the linear equations not specifying all of the original constraints. For example it is clear setting all the cube values to zero would satisfy the linear equations, but not the original equations.

So choosing some non-zero tuple value, the original quadratic equation can be used to constrain the final 2 freedoms (this constraint looks like a quadratic form equal to a constant). Therefore this gives a unique solution up to an overall sign.

## Properties of solutions to the cube equations

The cube equations have some interesting properties.

### New solution with overall sign flip

Since all the tuple values A1,A2,etc. are a sum of terms "quadratic" in the cube values, if we change the sign of all the cube values we get another cube solution with the same tuple values.

### Equivalence Transformations

Given a cube, there are some simple transformations we can do to the values which preserves two of the three tuples, and changes the third in a simple way.

In particular for this subsection, we will look at transformations which preserve Tuple1,Tuple2 and only change Tuple3. Given a cube, any new tuple we can obtain this way for Tuple3 is said to be "equivalent" to the original Tuple3. The full set of forms related in this way is called an "equivalence class".

Here is an example transformation that preserves Tuple1, and Tuple2, but changes Tuple3 in a simple way.

swap the values according to (a',b',c',d', e',f',g',h') = (c,d,-a,-b, g,h,-e,-f) A1' = b' c' - a' d' = d (-a) - c (-b) = bc - ad = A1 A2' = c' e' - a' g' = (-a) g - c (-e) = ce - ag = A2 A3' = b' e' - a' f' = d g - c h = C3 and so on... it is found that A1',B1',C1' = A1,B1,C1 A2',B2',C2' = A2,B2,C2 A3',B3',C3' = C3,-B3,A3

another operation preserving Tuple1,Tuple2 and changing Tuple3 is

given any integer n (a',b',c',d', e',f',g',h') = (a,b,c+an,d+bn, e,f,g+en,h+fn) C1' = f' g' - e' h' = f (g+en) - e (h+fn) = fg - eh = C1 C2' = d' f' - b' h' = (d+bn) f - b (h+fn) = df - bh = C2 C3' = d' g' - c' h' = (d+bn)(g+en) - (c+an)(h+fn) = (dg-ch) + n(-ah + bg - cf + de) + n^2(be - af) = C3 + n B3 + n^2 A3 and so on... it is found that A1',B1',C1' = A1,B1,C1 A2',B2',C2' = A2,B2,C2 A3',B3',C3' = A3, B3 + 2n A3, C3 + n B3 + n^2 A3

The previous two manipulations can be rephrased nicely in the language of linear algebra

first |a' b' e' f'| = | 0 1| |a b e f| |c' d' g' h'| |-1 0| |c d g h| | A3' B3'/2| = | 0 1| | A3 B3/2| |0 -1| |B3'/2 C3' | |-1 0| |B3/2 C3 | |1 0| second |a' b' e' f'| = |1 0| |a b e f| |c' d' g' h'| |n 1| |c d g h| | A3' B3'/2| = |1 0| | A3 B3/2| |1 n| |B3'/2 C3' | |n 1| |B3/2 C3 | |0 1|

These two manipulations can be combined, and repeated, leading to the following general case

modify with any matrix such that ru - st = 1 |a' b' e' f'| = |r s| |a b e f| |c' d' g' h'| |t u| |c d g h| | A3' B3'/2| = |r s| | A3 B3/2| |r t| |B3'/2 C3' | |t u| |B3/2 C3 | |s u|

This freedom in the tuple is precisely the freedom in the equivalance relation mentioned above.

A routine to do a general matrix transformation on the cube like this
is provided in the algocomp library: `transform_cube(cube, r,s,t,u)`

.

## Algebra details specific to the case of interest

From the constraint the discriminant is negative a prime number p, and the following constraint for any cube solution:

B1^2 - 4 A1 C1 = B2^2 - 4 A2 C2 = B3^2 - 4 A3 C3 = - p

We can see that no tuple value can be zero, as that would require p to have a non-trivial factor.

Similarly, we can see that gcd(B1,A1) = gcd(B1,C1) = 1, and likewise for the other two forms.