grounding-choice
At its defaults it draws one system, 60 ways to remove its kernel, and 10.5× between them. A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the preferential 60: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 53, of degree 2, gives κ = 926.22; grounding vertex 3, of degree 17, gives 88.226 — a factor of 10.5 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
grounding-choice is one function in lib/figures/graphlap.js —
the matrix a graph makes — row sums that are exactly zero, a count that is a threshold, and a partition with no vector. Everything below came out of it during this build, at
arguments taken from the essays rather than invented for this page. A figure here is the
figure a reader meets in an essay, and if the generator changes, this page changes with it.
At its defaults
Drawn even though every essay passes arguments — which on this site is every essay, at 100% of placements since the standard pass. A default nothing exercises is a trap for the next essay to call this with none, and this is the page where a default that has drifted from the figures around it becomes visible.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the preferential 60: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 53, of degree 2, gives κ = 926.22; grounding vertex 3, of degree 17, gives 88.226 — a factor of 10.5 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
family: "star", n: 30
The arguments are the ones A bound with a square root in it passes. A value drawn at the generator's defaults instead would be a picture no essay asked for and no assertion has been run against.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the star 30: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 1, of degree 1, gives κ = 898; grounding vertex 0, of degree 29, gives 1 — a factor of 898 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
family: "grid", n: 36
The arguments are the ones A count that comes out of a determinant passes. A value drawn at the generator's defaults instead would be a picture no essay asked for and no assertion has been run against.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the grid 6×6: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 5, of degree 2, gives κ = 275.34; grounding vertex 15, of degree 4, gives 96.565 — a factor of 2.851 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
family: "barbell", n: 20
The arguments are the ones A partition decided in the last digit passes. A value drawn at the generator's defaults instead would be a picture no essay asked for and no assertion has been run against.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the barbell 10: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 11, of degree 9, gives κ = 153.78; grounding vertex 10, of degree 10, gives 118.99 — a factor of 1.292 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
family: "skewed", n: 60
The arguments are the ones The vertex nobody solves for passes. A value drawn at the generator's defaults instead would be a picture no essay asked for and no assertion has been run against.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the preferential 60: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 53, of degree 2, gives κ = 926.22; grounding vertex 3, of degree 17, gives 88.226 — a factor of 10.5 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
family: "path", n: 30
The arguments are the ones The vertex nobody solves for passes. A value drawn at the generator's defaults instead would be a picture no essay asked for and no assertion has been run against.
A Laplacian is singular, so a solve has to remove its kernel, and the usual way is to delete one row and column — grounding a vertex, in the electrical reading. Which vertex is a free parameter that no account of the method mentions, and it is set by whichever index the code happens to drop. Every point here is one choice on the path 30: the condition number of the resulting positive definite matrix against the degree of the vertex removed. Grounding vertex 0, of degree 1, gives κ = 1407.1; grounding vertex 14, of degree 2, gives 385.82 — a factor of 3.647 between two ways of solving the identical problem. The trend is that a high-degree vertex is the better ground, which has a reading: grounding a vertex fixes its potential, and fixing the potential of something the rest of the graph is strongly attached to constrains more of the graph.
What it checked while drawing
Every figure above asserted its own claims on the way to being drawn, and a claim that failed
would have failed the build rather than drawn a wrong picture. Those assertions used to leave
no trace at all: a passing one returned true and the only evidence the figure had
checked anything was that nothing crashed. The list below is what they actually said, collected
by running this generator with an observer installed — not a description of
what it is believed to check.
10 distinct claims across 6 sets of arguments, grouped below by shape — because most of them are one sentence with a different number in it, and how many separate times that sentence was put to the test is the informative part.
a connected graph, which is what grounding one vertex needs
a family the library builds
a graph small enough to ground at every vertex in turn
a graph with at least two vertices
a planted partition with no isolated vertex
a positive weight
every endpoint inside the vertex set
every grounded Laplacian is positive definite
no edge given twice
no self-loop
Against the rule
The rule does not apply to it. It factorises nothing, so there is no residual it could be withholding. That is worth stating rather than leaving blank: a site that reported the rule as satisfied by every generator would be counting mostly generators the rule never reached.
Across the library: the rule bites on 214
of 382 generators —
194 print a residual and
20 are exempt with a published reason;
168 factorise nothing.
Read from lib/residual-rule.js, which is the same body the gate enforces from,
and the gate's last check fails the build if this page and it disagree about any generator.
Where it is called
Changing this generator changes every figure on this list. That is what makes the list worth publishing rather than keeping in a check script.
A bound with a square root in it
Cheeger's inequality brackets a graph's best cut between λ₂/2 and √(2λ₂). The lower bound is attained exactly. The upper one is loose by a factor of fourteen — on the one graph in the census with a real bottleneck, which is the shape it is always quoted about.
The matrix that is a graphA count that comes out of a determinant
The number of spanning trees of a graph is the determinant of its grounded Laplacian, so it is a whole number known in advance. The elimination that computes it is backward stable at every size — and from sixteen vertices the answer is wrong, because the count has seventeen digits and a binary64 has sixteen.
The matrix that is a graphA distance computed by a solve
Effective resistance is the one quantity in this field with no combinatorial route to it — it is defined by a linear system. On a small unweighted graph the answer is a ratio of two integers, so for once the error is known rather than estimated, and every resistance in a graph has to add up to a number fixed in advance.
The matrix that is a graphA partition decided in the last digit
On a graph with a symmetry there is no Fiedler vector — there is a plane, and every vector in it is an exact eigenvector. Twenty-four runs with the edge weights nudged by 10⁻¹² return ten different partitions of a cycle and, on a hypercube, two different qualities of answer.
The matrix that is a graphA preconditioner that is a tree
Every eigenvalue of a tree-preconditioned Laplacian is at least one and at most the total stretch — a combinatorial integer with no arithmetic in it. Measured, the bound is two to four times loose, and on a grid the preconditioner makes the conditioning worse by a factor of 1.85 at every size.
The matrix that is a graphEliminating a vertex is a graph operation
Gaussian elimination on a Laplacian deletes a vertex and joins its neighbours into a clique with conductances wᵢwⱼ over Σw. The matrix that remains is still a graph — symmetric, zero row sums, nonpositive off the diagonal — and the ordering decides whether the fill is thirty-one edges or four hundred and sixty-five.
The matrix that is a graphThe spectrum is not the graph
Two graphs on six vertices with the same Laplacian characteristic polynomial — as integer polynomials, not to fourteen digits. One contains a triangle; the other is bipartite. Every method in this field that reads only the spectrum is answering about the class.
The matrix that is a graphThe vector that has to be rounded
A spectral partition is an eigenvector, and an eigenvector is a real vector. The answer wanted is a subset. Something has to turn one into the other, and the something is a heuristic applied after the linear algebra has finished.
The matrix that is a graphThe vertex nobody solves for
A Laplacian is singular, so every solve with one has to remove its kernel first. There are three ways, they agree to fourteen digits, and the one everybody uses carries a free parameter that no account of the method mentions and that moves the condition number by nine hundred.
The matrix that is a graphTwo Laplacians of one graph
D − A and D^{-1/2}(D − A)D^{-1/2} are built from the same object, are not similar to each other, and answer different questions. On a graph whose degrees are equal they coincide. On one whose degrees span an order of magnitude their second eigenvalues are sixteen times apart.