Rotation matrix - Wikipedia
https://en.wikipedia.org/wiki/Rotation_matrixGiven a 3 × 3 rotation matrix R, a vector u parallel to the rotation axis must satisfy =, since the rotation of u around the rotation axis must result in u.The equation above may be solved for u which is unique up to a scalar factor unless R = I.. Further, the equation may be rewritten = =, which shows that u lies in the null space of R − I.
Matrix calculator
https://matrixcalc.org/enMatrix addition, multiplication, inversion, determinant and rank calculation, transposing, bringing to diagonal, triangular form, exponentiation, LU Decomposition, Singular Value Decomposition (SVD), solving of systems of linear equations with solution steps