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8 changed files with 143 additions and 2310 deletions

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@ -65,12 +65,8 @@ mutable struct ConstructionViewer
}
""")
# load Ganja.js. for an automatically updated web-hosted version, load from
#
# https://unpkg.com/ganja.js
#
# instead
loadjs!(win, "http://localhost:8000/ganja-1.0.204.js")
# load Ganja.js
loadjs!(win, "https://unpkg.com/ganja.js")
# create global functions and variables
script!(win, """
@ -127,24 +123,12 @@ mutable struct ConstructionViewer
end
end
mprod(v, w) =
v[1]*w[1] + v[2]*w[2] + v[3]*w[3] + v[4]*w[4] - v[5]*w[5]
function display!(viewer::ConstructionViewer, elements::Matrix)
# load elements
elements_full = []
for elt in eachcol(elements)
if mprod(elt, elt) < 0.5
elt_full = [0; elt; fill(0, 26)]
else
# `elt` is a spacelike vector, representing a generalized sphere, so we
# take its Hodge dual before passing it to Ganja.js. the dual represents
# the same generalized sphere, but Ganja.js only displays planes when
# they're represented by vectors in grade 4 rather than grade 1
elt_full = [fill(0, 26); -elt[5]; -elt[4]; elt[3]; -elt[2]; elt[1]; 0]
end
push!(elements_full, elt_full)
end
elements_full = [
[0; elt; fill(0, 26)]
for elt in eachcol(elements)
]
@js viewer.win elements = $elements_full.map((elt) -> @new CGA3(elt))
# generate palette. this is Gadfly's `default_discrete_colors` palette,

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@ -28,25 +28,6 @@
CGA3.inline(() => Math.sqrt(0.5)*(-1e1 - 1e2 + 1e3 + 1e5))(),
CGA3.inline(() => -Math.sqrt(3)*1e4 + Math.sqrt(2)*1e5)()
];
/*
these blocks of commented-out code can be used to confirm that a spacelike
vector and its Hodge dual represent the same generalized sphere
*/
/*let elements = [
CGA3.inline(() => Math.sqrt(0.5)*!( 1e1 + 1e2 + 1e3 + 1e5))(),
CGA3.inline(() => Math.sqrt(0.5)*!( 1e1 - 1e2 - 1e3 + 1e5))(),
CGA3.inline(() => Math.sqrt(0.5)*!(-1e1 + 1e2 - 1e3 + 1e5))(),
CGA3.inline(() => Math.sqrt(0.5)*!(-1e1 - 1e2 + 1e3 + 1e5))(),
CGA3.inline(() => !(-Math.sqrt(3)*1e4 + Math.sqrt(2)*1e5))()
];*/
/*let elements = [
CGA3.inline(() => 1e1 + 1e5)(),
CGA3.inline(() => 1e2 + 1e5)(),
CGA3.inline(() => 1e3 + 1e5)(),
CGA3.inline(() => -1e4 + 1e5)(),
CGA3.inline(() => Math.sqrt(0.5)*(1e1 + 1e2 + 1e3 + 1e5))(),
CGA3.inline(() => Math.sqrt(0.5)*!(1e1 + 1e2 + 1e3 - 0.01e4 + 1e5))()
];*/
// set up palette
var colorIndex;
@ -85,7 +66,6 @@
// de-noise
for (let k = 6; k < elements[n].length; ++k) {
/*for (let k = 0; k < 26; ++k) {*/
elements[n][k] = 0;
}
}

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@ -1,146 +0,0 @@
module Engine
using LinearAlgebra
using SparseArrays
using Random
export rand_on_shell, Q, DescentHistory, realize_gram
# === guessing ===
sconh(t, u) = 0.5*(exp(t) + u*exp(-t))
function rand_on_sphere(rng::AbstractRNG, ::Type{T}, n) where T
out = randn(rng, T, n)
tries_left = 2
while dot(out, out) < 1e-6 && tries_left > 0
out = randn(rng, T, n)
tries_left -= 1
end
normalize(out)
end
##[TO DO] write a test to confirm that the outputs are on the correct shells
function rand_on_shell(rng::AbstractRNG, shell::T) where T <: Number
space_part = rand_on_sphere(rng, T, 4)
rapidity = randn(rng, T)
sig = sign(shell)
[sconh(rapidity, sig)*space_part; sconh(rapidity, -sig)]
end
rand_on_shell(rng::AbstractRNG, shells::Array{T}) where T <: Number =
hcat([rand_on_shell(rng, sh) for sh in shells]...)
rand_on_shell(shells::Array{<:Number}) = rand_on_shell(Random.default_rng(), shells)
# === elements ===
plane(normal, offset) = [normal; offset; offset]
function sphere(center, radius)
dist_sq = dot(center, center)
return [
center / radius;
0.5 * ((dist_sq - 1) / radius - radius);
0.5 * ((dist_sq + 1) / radius - radius)
]
end
# === Gram matrix realization ===
# the Lorentz form
Q = diagm([1, 1, 1, 1, -1])
# the difference between the matrices `target` and `attempt`, projected onto the
# subspace of matrices whose entries vanish at each empty index of `target`
function proj_diff(target::SparseMatrixCSC{T, <:Any}, attempt::Matrix{T}) where T
J, K, values = findnz(target)
result = zeros(size(target)...)
for (j, k, val) in zip(J, K, values)
result[j, k] = val - attempt[j, k]
end
result
end
# a type for keeping track of gradient descent history
struct DescentHistory{T}
scaled_loss::Array{T}
slope::Array{T}
stepsize::Array{T}
backoff_steps::Array{Int64}
function DescentHistory{T}(
scaled_loss = Array{T}(undef, 0),
slope = Array{T}(undef, 0),
stepsize = Array{T}(undef, 0),
backoff_steps = Int64[]
) where T
new(scaled_loss, slope, stepsize, backoff_steps)
end
end
# seek a matrix `L` for which `L'QL` matches the sparse matrix `gram` at every
# explicit entry of `gram`. use gradient descent starting from `guess`
function realize_gram(
gram::SparseMatrixCSC{T, <:Any},
guess::Matrix{T};
scaled_tol = 1e-30,
target_improvement = 0.5,
init_stepsize = 1.0,
backoff = 0.9,
max_descent_steps = 600,
max_backoff_steps = 110
) where T <: Number
# start history
history = DescentHistory{T}()
# scale tolerance
scale_adjustment = sqrt(T(nnz(gram)))
tol = scale_adjustment * scaled_tol
# initialize variables
stepsize = init_stepsize
L = copy(guess)
# do gradient descent
Δ_proj = proj_diff(gram, L'*Q*L)
loss = norm(Δ_proj)
for step in 1:max_descent_steps
# stop if the loss is tolerably low
if loss < tol
break
end
# find negative gradient of loss function
neg_grad = 4*Q*L*Δ_proj
slope = norm(neg_grad)
# store current position, loss, and slope
L_last = L
loss_last = loss
push!(history.scaled_loss, loss / scale_adjustment)
push!(history.slope, slope)
# find a good step size using backtracking line search
push!(history.stepsize, 0)
push!(history.backoff_steps, max_backoff_steps)
for backoff_steps in 0:max_backoff_steps
history.stepsize[end] = stepsize
L = L_last + stepsize * neg_grad
Δ_proj = proj_diff(gram, L'*Q*L)
loss = norm(Δ_proj)
improvement = loss_last - loss
if improvement >= target_improvement * stepsize * slope
history.backoff_steps[end] = backoff_steps
break
end
stepsize *= backoff
end
end
# return the factorization and its history
push!(history.scaled_loss, loss / scale_adjustment)
L, history
end
end

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@ -1,79 +0,0 @@
include("Engine.jl")
using SparseArrays
using AbstractAlgebra
using PolynomialRoots
# initialize the partial gram matrix for a sphere inscribed in a regular
# tetrahedron
J = Int64[]
K = Int64[]
values = BigFloat[]
for j in 1:8
for k in 1:8
filled = false
if j == k
push!(values, 1)
filled = true
elseif (j == 1 || k == 1)
push!(values, 0)
filled = true
elseif (j == 2 || k == 2)
push!(values, -1)
filled = true
end
if filled
push!(J, j)
push!(K, k)
end
end
end
append!(J, [6, 4, 6, 5, 7, 5, 7, 3, 8, 3, 8, 4])
append!(K, [4, 6, 5, 6, 5, 7, 3, 7, 3, 8, 4, 8])
append!(values, fill(-1, 12))
#= make construction rigid
append!(J, [3, 4, 4, 5])
append!(K, [4, 3, 5, 4])
append!(values, fill(-0.5, 4))
=#
gram = sparse(J, K, values)
# set initial guess (random)
## Random.seed!(58271) # stuck; step size collapses on step 48
## Random.seed!(58272) # good convergence
## Random.seed!(58273) # stuck; step size collapses on step 18
## Random.seed!(58274) # stuck
## Random.seed!(58275) #
## guess = Engine.rand_on_shell(fill(BigFloat(-1), 8))
# set initial guess
#=
guess = hcat(
Engine.plane(BigFloat[0, 0, 1], BigFloat(0)),
Engine.sphere(BigFloat[0, 0, 0], BigFloat(1//2)),
Engine.plane(BigFloat[1, 0, 0], BigFloat(1)),
Engine.plane(BigFloat[cos(2pi/3), sin(2pi/3), 0], BigFloat(1)),
Engine.plane(BigFloat[cos(-2pi/3), sin(-2pi/3), 0], BigFloat(1)),
Engine.sphere(BigFloat[-1, 0, 0], BigFloat(1//5)),
Engine.sphere(BigFloat[cos(-pi/3), sin(-pi/3), 0], BigFloat(1//5)),
Engine.sphere(BigFloat[cos(pi/3), sin(pi/3), 0], BigFloat(1//5))
)
=#
guess = hcat(
Engine.plane(BigFloat[0, 0, 1], BigFloat(0)),
Engine.sphere(BigFloat[0, 0, 0], BigFloat(0.9)),
Engine.plane(BigFloat[1, 0, 0], BigFloat(1)),
Engine.plane(BigFloat[cos(2pi/3), sin(2pi/3), 0], BigFloat(1)),
Engine.plane(BigFloat[cos(-2pi/3), sin(-2pi/3), 0], BigFloat(1)),
Engine.sphere(4//3*BigFloat[-1, 0, 0], BigFloat(1//3)),
Engine.sphere(4//3*BigFloat[cos(-pi/3), sin(-pi/3), 0], BigFloat(1//3)),
Engine.sphere(4//3*BigFloat[cos(pi/3), sin(pi/3), 0], BigFloat(1//3))
)
# complete the gram matrix using gradient descent
L, history = Engine.realize_gram(gram, guess, max_descent_steps = 200)
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)
println("\nSteps: ", size(history.stepsize, 1))
println("Loss: ", history.scaled_loss[end], "\n")

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@ -0,0 +1,137 @@
using LinearAlgebra
using SparseArrays
using AbstractAlgebra
using PolynomialRoots
# testing Gram matrix recovery using a homemade gradient descent routine
# === gradient descent ===
# the difference between the matrices `target` and `attempt`, projected onto the
# subspace of matrices whose entries vanish at each empty index of `target`
function proj_diff(target, attempt)
J, K, values = findnz(target)
result = zeros(size(target)...)
for (j, k, val) in zip(J, K, values)
result[j, k] = val - attempt[j, k]
end
result
end
# === example ===
# the Lorentz form
Q = diagm([1, 1, 1, 1, -1])
# initialize the partial gram matrix for an arrangement of seven spheres in
# which spheres 1 through 5 are mutually tangent, and spheres 3 through 7 are
# also mutually tangent
J = Int64[]
K = Int64[]
values = BigFloat[]
for j in 1:7
for k in 1:7
if (j <= 5 && k <= 5) || (j >= 3 && k >= 3)
push!(J, j)
push!(K, k)
push!(values, j == k ? 1 : -1)
end
end
end
gram = sparse(J, K, values)
# set the independent variable
#
# using gram[6, 2] or gram[7, 1] as the independent variable seems to stall
# convergence, even if its value comes from a known solution, like
#
# gram[6, 2] = 0.9936131705272925
#
indep_val = -9//5
gram[6, 1] = BigFloat(indep_val)
gram[1, 6] = gram[6, 1]
# in this initial guess, the mutual tangency condition is satisfied for spheres
# 1 through 5
guess = sqrt(0.5) * BigFloat[
1 1 -1 -1 0 -0.1 0.3;
1 -1 1 -1 0 -0.5 0.4;
1 -1 -1 1 0 0.1 -0.2;
0 0 0 0 -sqrt(6) 0.3 -0.2;
1 1 1 1 2 0.2 0.1;
]
# search parameters
steps = 600
line_search_max_steps = 100
init_stepsize = BigFloat(1)
step_shrink_factor = BigFloat(0.5)
target_improvement_factor = BigFloat(0.5)
# complete the gram matrix using gradient descent
loss_history = Array{BigFloat}(undef, steps + 1)
stepsize_history = Array{BigFloat}(undef, steps)
line_search_depth_history = fill(line_search_max_steps, steps)
stepsize = init_stepsize
L = copy(guess)
Δ_proj = proj_diff(gram, L'*Q*L)
loss = norm(Δ_proj)
for step in 1:steps
# find negative gradient of loss function
neg_grad = 4*Q*L*Δ_proj
slope = norm(neg_grad)
# store current position and loss
L_last = L
loss_last = loss
loss_history[step] = loss
# find a good step size using backtracking line search
for line_search_depth in 1:line_search_max_steps
stepsize_history[step] = stepsize
global L = L_last + stepsize * neg_grad
global Δ_proj = proj_diff(gram, L'*Q*L)
global loss = norm(Δ_proj)
improvement = loss_last - loss
if improvement >= target_improvement_factor * stepsize * slope
line_search_depth_history[step] = line_search_depth
break
end
global stepsize *= step_shrink_factor
end
end
completed_gram = L'*Q*L
loss_history[steps + 1] = loss
println("Completed Gram matrix:\n")
display(completed_gram)
println("\nLoss: ", loss, "\n")
# === algebraic check ===
R, gens = polynomial_ring(AbstractAlgebra.Rationals{BigInt}(), ["x", "t₁", "t₂", "t₃"])
x = gens[1]
t = gens[2:4]
S, u = polynomial_ring(AbstractAlgebra.Rationals{BigInt}(), "u")
M = matrix_space(R, 7, 7)
gram_symb = M(R[
1 -1 -1 -1 -1 t[1] t[2];
-1 1 -1 -1 -1 x t[3]
-1 -1 1 -1 -1 -1 -1;
-1 -1 -1 1 -1 -1 -1;
-1 -1 -1 -1 1 -1 -1;
t[1] x -1 -1 -1 1 -1;
t[2] t[3] -1 -1 -1 -1 1
])
rank_constraints = det.([
gram_symb[1:6, 1:6],
gram_symb[2:7, 2:7],
gram_symb[[1, 3, 4, 5, 6, 7], [1, 3, 4, 5, 6, 7]]
])
# solve for x and t
x_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[1], [2], [indep_val]))
t₂_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[3], [2], [indep_val]))
x_vals = PolynomialRoots.roots(x_constraint.coeffs)
t₂_vals = PolynomialRoots.roots(t₂_constraint.coeffs)

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@ -1,86 +0,0 @@
include("Engine.jl")
using SparseArrays
using AbstractAlgebra
using PolynomialRoots
using Random
# initialize the partial gram matrix for an arrangement of seven spheres in
# which spheres 1 through 5 are mutually tangent, and spheres 3 through 7 are
# also mutually tangent
J = Int64[]
K = Int64[]
values = BigFloat[]
for j in 1:7
for k in 1:7
if (j <= 5 && k <= 5) || (j >= 3 && k >= 3)
push!(J, j)
push!(K, k)
push!(values, j == k ? 1 : -1)
end
end
end
gram = sparse(J, K, values)
# set the independent variable
#
# using gram[6, 2] or gram[7, 1] as the independent variable seems to stall
# convergence, even if its value comes from a known solution, like
#
# gram[6, 2] = 0.9936131705272925
#
indep_val = -9//5
gram[6, 1] = BigFloat(indep_val)
gram[1, 6] = gram[6, 1]
# in this initial guess, the mutual tangency condition is satisfied for spheres
# 1 through 5
Random.seed!(50793)
guess = hcat(
sqrt(1/BigFloat(2)) * BigFloat[
1 1 -1 -1 0;
1 -1 1 -1 0;
1 -1 -1 1 0;
0 0 0 0 -sqrt(BigFloat(6));
1 1 1 1 2;
] + 0.2*Engine.rand_on_shell(fill(BigFloat(-1), 5)),
Engine.rand_on_shell(fill(BigFloat(-1), 2))
)
# complete the gram matrix using gradient descent
L, history = Engine.realize_gram(gram, guess)
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)
println("\nSteps: ", size(history.stepsize, 1))
println("Loss: ", history.scaled_loss[end], "\n")
# === algebraic check ===
R, gens = polynomial_ring(AbstractAlgebra.Rationals{BigInt}(), ["x", "t₁", "t₂", "t₃"])
x = gens[1]
t = gens[2:4]
S, u = polynomial_ring(AbstractAlgebra.Rationals{BigInt}(), "u")
M = matrix_space(R, 7, 7)
gram_symb = M(R[
1 -1 -1 -1 -1 t[1] t[2];
-1 1 -1 -1 -1 x t[3]
-1 -1 1 -1 -1 -1 -1;
-1 -1 -1 1 -1 -1 -1;
-1 -1 -1 -1 1 -1 -1;
t[1] x -1 -1 -1 1 -1;
t[2] t[3] -1 -1 -1 -1 1
])
rank_constraints = det.([
gram_symb[1:6, 1:6],
gram_symb[2:7, 2:7],
gram_symb[[1, 3, 4, 5, 6, 7], [1, 3, 4, 5, 6, 7]]
])
# solve for x and t
x_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[1], [2], [indep_val]))
t₂_constraint = 25//16 * to_univariate(S, evaluate(rank_constraints[3], [2], [indep_val]))
x_vals = PolynomialRoots.roots(x_constraint.coeffs)
t₂_vals = PolynomialRoots.roots(t₂_constraint.coeffs)

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@ -1,44 +0,0 @@
include("Engine.jl")
using SparseArrays
using AbstractAlgebra
using PolynomialRoots
using Random
# initialize the partial gram matrix for a sphere inscribed in a regular
# tetrahedron
J = Int64[]
K = Int64[]
values = BigFloat[]
for j in 1:5
for k in 1:5
push!(J, j)
push!(K, k)
if j == k
push!(values, 1)
elseif (j <= 4 && k <= 4)
push!(values, -1/BigFloat(3))
else
push!(values, -1)
end
end
end
gram = sparse(J, K, values)
# set initial guess
Random.seed!(99230)
guess = sqrt(1/BigFloat(3)) * BigFloat[
1 1 -1 -1 0
1 -1 1 -1 0
1 -1 -1 1 0
1 1 1 1 -2
1 1 1 1 1
] + 0.2*Engine.rand_on_shell(fill(BigFloat(-1), 5))
# complete the gram matrix using gradient descent
L, history = Engine.realize_gram(gram, guess)
completed_gram = L'*Engine.Q*L
println("Completed Gram matrix:\n")
display(completed_gram)
println("\nSteps: ", size(history.stepsize, 1))
println("Loss: ", history.scaled_loss[end], "\n")