Thread: rotation matrix
View Single Post
Old 2022-09-22, 03:01   #12
bhelmes's Avatar
Mar 2016

1101000002 Posts
Default Is it possible to calculate one belonging rot. matrix from a vector ?

Originally Posted by Dr Sardonicus View Post
No, sir. If M is a 2x2 matrix, scalar multiplication of M by k produces a matrix with determinant k^2*det(M). Thus, if M is a nonsingular 2x2 matrix, det((1/det(M))M) is 1/det(M).

If M is 2x2 and det(M) is not a square, no scalar multiple of M will have determinant 1.
A peaceful, early morning, especially for Dr Sardonicus,

let: p=31, u1=2, v1=3; so that the norm (u1,v1)=u1²+v1²=13=12⁻¹ mod 31 and 13²=20⁻¹ mod 31

Is it possible from linear algebra to calculate one belonging rotation matrix from this vector ?

The calculated target is:
(27,2)* (2)=(5)
(-2,27) (3)=(9)

(14,17)* (2)=(4)
(-17,14) (3)=(11)

(24,8)* (2)=(6)
(-8,24) (3)=(15)


(the red coloured boxes right, all calculations checked and it seems to be all right.)

My first try:
Let p=31

Let M1=13*M1*=
(a*, b*)
(-b*, a*)

1. with det (M1)=1=det (13² * det (M1*)) so that det (M1*)=20 mod 31, therefore a*²+b*²=20
2. with M1*(u1,v1)=(u2,v2) with norm (u1,v1)=(u2,v2)=u1²+v1²=u2²+v2²=13 mod p

so that

(a*, b*) = (u2)
(-b*, a*) (v2)

This is more a fragment and should point in one direction, and as it is too late for me,
I hope that some one could finish the calculation.

bhelmes is offline   Reply With Quote