yes cianfa72, if b is in that column space then so is -b, so there exists x with (A-I)x = -b, so Ax-x = -b, so Ax+b = x.
Thank you Orodruin; with your guidance, I have come up with a version for all dimensions, generalizing the results from dim 3: (I have not written down all the proofs, so errors are certainly possible.)
(Oriented) Isometries of Euclidean space:
Every orientation preserving isometry of Euclidean space is an orthogonal product of plane rotations, and a translation parallel to the intersection of the axes of the rotations.
More precisely:
Rotations
If p is a point of a Euclidean 2-plane ∏, a rotation centered at p is an orientation preserving isometry of E that fixes p.
If a Euclidean space E is a product of ≥1 orthogonal 2-planes ∏j and another orthogonal space C, the product of a non trivial rotation in each ∏j, with the identity map on C, is called a (non-trivial) rotation of E, with axis (fixed set) C. (Every matrix in SO(n) defines such a map of R^n.)
The fixed set of a rotation is always non empty; the trivial rotation fixes all of E. A product of non trivial rotations, (possible only on an even dimensional space E), fixes only a point.
Theorem:
An isometry of E is a rotation if and only if it has a fixed point.
Translations
A translation is an isometry T such that for any two points p,q, the points p,q,T(p),T(q), all lie in the same plane, and the oriented segments pT(p), and qT(q), are opposite sides of a parallelogram. A translation is entirely determined by any point p and its image T(p). In particular a translation with a fixed point is the identity.
An isometry A is a translation if and only if there is a point p such that, if T is the translation from p to A(p), then T^-1A is the identity map of E, if and only if E (of positive dimension), is the union of the invariant lines of A.
General (oriented) isometries:
Theorem:
Every orientation preserving isometry A of E is uniquely a product A = TR of a (possibly trivial) rotation R and a (possibly trivial) translation T that leaves the axis of R invariant. The axis of R is called the axis, or center, of A. T is the (intrinsic) “translation part” of A, and R is the (intrinsic) “rotation part” of A.
Given any isometry A of E, the axis of A is the unique maximal subspace C on which A restricts to a (possibly trivial) translation T. C is always non empty. If A has a fixed point, its axis C is the union of all fixed points of A. If A has no fixed points, its axis C is the union of all invariant lines of A.
Given A, and hence C and T, the product T^-1A is the identity on C, hence equals a rotation R. Then A = TR is the unique decomposition of A into a rotation R, and a translation T leaving the axis of R invariant.
Next: how to represent such isometries with matrices, and translation vectors, after choosing a base point 0.
Briefly, then an isometry A is uniquely a product tr, where r is a rotation with axis containing 0, and t is a translation not necessarily leaving the axis of r invariant. If we then decompose t into two translations, t = t1.t0, where t0 is orthogonal to the axis of r, and t1 is parallel to that axis, then the product t0.r is again a rotation R, with axis C parallel to the axis of r, and t1 = T is then a translation parallel to the axis C of R = t0r. hence we have decomposed A into a product A = tr = t1.(t0.r) = TR, as above.
Here r is given by a matrix in SO(n), and t is given as translation by a vector in R^n, which makes computations doable. But note that even though the expression A = tr is unique, this t is not usually the (intrinsic) translation part of A, and r is not usually the (intrinsic) rotation part of A.
When A = tr, still r does contain a lot of information about R, e.g. r is a product of the same number of non trivial plane rotations as R, and its axis is parallel to that of R and of the same dimension. Moreover r=Id if and only if t0 = Id, if and only if R=Id, if and only if A = t=T; i.e. A = tr is a translation iff r = Id. Moreover, if t = Id, then A = r = R is a rotation. However it is possible for A = tr, to be a rotation even if t ≠ Id. Indeed this happens iff t is a translation orthogonal to the axis of r.