Commit 2020-06-30 03:05 791744b0
View on Github →feat(analysis/normed_space/real_inner_product,geometry/euclidean): inner products of weighted subtractions (#3203)
Express the inner product of two weighted sums, where the weights in
each sum add to 0, in terms of the norms of pairwise differences.
Thus, express inner products for vectors expressed in terms of
weighted_vsub
and distances for points expressed in terms of
affine_combination
.
This is a general form of the standard formula for a distance between
points expressed in terms of barycentric coordinates: if the
difference between the normalized barycentric coordinates (with
respect to triangle ABC) for two points is (x, y, z) then the squared
distance between them is -(a^2 yz + b^2 zx + c^2 xy).
Although some of the lemmas are given with the two vectors expressed
as sums over different indexed families of vectors or points (since
nothing in the statement or proof depends on them being the same), I
expect almost all uses will be in cases where the two indexed families
are the same and only the weights vary.