Euclidean similarities
A bijection of ℝⁿ uniformly scaling all distances by s>0 is a similarity and has form T(x)=sQx+b with Q orthogonal.
Key knowledge
A bijection of ℝⁿ uniformly scaling all distances by s>0 is a similarity and has form T(x)=sQx+b with Q orthogonal. It preserves angles and ratios of nonzero distances; distances are multiplied by s. Similarities form a group containing the isometries s=1.
Conditions and scope
- Use finite-dimensional real Euclidean space; angles require nonzero displacement vectors and ratios require a nonzero denominator.
- The scale of a composition is the product of its scales; the inverse scale is 1/s.
Sources · 1
References
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Evidence and review
Section 1 Similarities definition, observations and matrix form, printed page 3; angle-invariance proposition and proof, printed page 4. Identity/group closure follows from the explicit composition and inverse observations.
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