Depiction of the ΔY- and YΔ-transformations applied to a graph.

In graph theory, ΔY- and YΔ-transformations (also written delta-wye and wye-delta) are a pair of operations on graphs. A ΔY-transformation replaces a triangle by a vertex of degree three; and conversely, a YΔ-transformation replaces a vertex of degree three by a triangle. The name for the operations derives from the shapes of the involved subgraphs, which look respectively like the letter Y and the Greek capital letter Δ.

A YΔ-transformation might create parallel edges, even if applied to a simple graph. For this reason ΔY- and YΔ-transformations are often considered as operations on multigraphs. On multigraphs both operations preserve the edge count and are exact inverses of each other.

In the context of simple graphs it is common to combine a YΔ-transformation with a subsequent normalization step that reduces any parallel edges to a single edge. This might no longer preserve the number of edges, nor be exactly reversible via a ΔY-transformation.

The YΔ-transformation can lead to multi-edges. Removing the multi-edges in a normalization step and applying a ΔY-transformation will then not result in the original graph.

Formal definition

Let ${\displaystyle G}$ be a graph (potentially a multigraph).

If the edges ${\displaystyle e_{12},e_{23},e_{31}}$ form a triangle ${\displaystyle \Delta }$ with vertices ${\displaystyle x_{1},x_{2},x_{3}}$, then a ΔY-transformation of ${\displaystyle G}$ at ${\displaystyle \Delta }$ removes ${\displaystyle e_{12},e_{23},e_{31}}$ and adds a new vertex ${\displaystyle y}$ adjacent to all of ${\displaystyle x_{1},x_{2},x_{3}}$.

If ${\displaystyle y}$ is a vertex of degree three with neighbors ${\displaystyle x_{1},x_{2},x_{3}}$, then a YΔ-transformation of ${\displaystyle G}$ at ${\displaystyle y}$ deletes ${\displaystyle y}$ and adds three new edges ${\displaystyle e_{12},e_{23},e_{31}}$ so that ${\displaystyle e_{ij}}$ connects ${\displaystyle x_{i}}$ and ${\displaystyle x_{j}}$. If the resulting graph is supposed to be a simple graph, then any resulting parallel edges are to be replaced by a single edge.

Relevance

ΔY- and YΔ-transformations are a tool both in pure graph theory as well as applications.

Both operations preserve a number of natural topological properties of graphs, such as being planar^[1] or linkless.^[2] This fact is used to compactly describe the forbidden minors of the associated graph classes as ΔY-families generated from a small number of graphs (see the section on ΔY-families below).

A particularly relevant application exists in electrical engineering in the study of three-phase power systems (see Y-Δ transform (electrical engineering)).^[3] In this context they are also known as star-triangle transformations and are a special case of star-mesh transformations.

ΔY-families

The six members of the Petersen family with edges denoting an application of a single YΔ- or ΔY-transformations.

The ΔY-family generated by a graph ${\displaystyle G}$ is the smallest family of graphs that contains ${\displaystyle G}$ and is closed under YΔ- and ΔY-transformations. Equivalently, it is constructed from ${\displaystyle G}$ by recursively applying these transformations until no new graph is generated. Since YΔ- and ΔY-transformation preserve edge count, if ${\displaystyle G}$ is a finite graph it generates a finite ΔY-family.

The ΔY-family generated by several graphs is the smallest family that contains all these graphs and is closed under YΔ- and ΔY-transformation.

Some notable families are generated in this way:

• the Petersen family (the six forbidden minors of the linkless graphs) is generated from the complete graph ${\displaystyle K_{6}}$.^[2]
• the Heawood family (the 78 conjectured forbidden minors for the 4-flat graphs) is generated from ${\displaystyle K_{7}}$ and ${\displaystyle K_{3,3,1,1}}$.^[4]

YΔY-reducible graphs

The graph formed by connecting a vertex to every degree-three vertex of a rhombic dodecahedron graph is linkless but not YΔY-reducible.

A graph is YΔY-reducible if it can be reduced to a single vertex by a sequence of ΔY- or YΔ-transformations and the following normalization steps:

• removing a loop,
• removing a parallel edge,
• removing a vertex of degree one,
• smoothing out a vertex of degree two, i.e., replacing it by a single edge between its two former neighbors.

The YΔY-reducible graphs form a minor closed family and therefore have a forbidden minor characterization (by the Robertson-Seymour theorem). The graphs of the Petersen family constitute some (but not all) of the excluded minors.^[5] In fact, already more than 68 billion excluded minor are known.^[6]

The YΔY-reducible graphs lie between the planar graphs and the linkless graphs: each planar graph is YΔY-reducible, while each YΔY-reducible graph is linkless. Both inclusions are strict: ${\displaystyle K_{5}}$ is not planar but YΔY-reducible, while the graph in the figure is not YΔY-reducible but linkless.^[5]

References