Ringel’s conjecture solved (for sufficiently large n)
A couple weeks ago and a few days after I heard an excellent lecture about it by Alexey Pokrovskiy in Oberwolfach, the paper A proof of Ringel’s Conjecture by Richard Montgomery, Alexey Pokrovskiy, and Benny Sudakov was arXived. Congratulations to Richard, Alexey, and Benny!
Here is the abstract:
A typical decomposition question asks whether the edges of some graph G can be partitioned into disjoint copies of another graph H. One of the oldest and best known conjectures in this area, posed by Ringel in 1963, concerns the decomposition of complete graphs into edge-disjoint copies of a tree. It says that any tree with n edges packs 2n+1 times into the complete graph . In this paper, we prove this conjecture for large n.
Previous posts: Test your intuition 43 – What will be the distribution of anabraists and algasisits; MIP*=RE. For Ringel’s circle conjecture, see this post.
Absorption and distributive absorption
First the authors construct approximate decomposition (they divide the problem into two cases) and then they apply a powerful method called absorption. Here is what they write:
To prove our ﬁnishing lemmas, we use an absorption strategy. Absorption has its origins in work by Erdős, Gyárfás and Pyber  and Krivelevich , but was codiﬁed by Rödl, Ruciński and Szemerédi  as a versatile technique for extending approximate results into exact ones. For both Case A and Case B we use a new implementation of distributive absorption, a method introduced by the ﬁrst author in .
Absorption is one of the most powerful current methods in extremal combinatorics. It is important in the construction of designs (See here, here, here, and here, and my own survey), and in the the solution of several other important conjectures.
Here are links to the pioneering papers mentioned above.
P. Erdős, A. Gyárfás and L. Pyber  Vertex coverings by monochromatic cycles and trees. Journal of Combinatorial Theory, Series B, 90–95, 1991.
M. Krivelevich  Triangle factors in random graphs. Combinatorics, Probability and Computing, 6: 337–347, 1997.
V. Rödl, A. Ruciński and E. Szemerédi  A dirac-type theorem for 3-uniform hypergraphs. Combin. Probab. Comput., 15:229–251, 2006.
And Montgomery’s paper on distributive absorption:
R. Montgomery  Spanning trees in random graphs, Advances in Mathematics, 2019
A related paper with an amusing name and an amusing abstract is: E. Szemerédi “Is laziness paying off? (`Absorbing’ method).” The abstract of the paper reads “We are going to mention some embedding problems. The main ideas of most of the solution of these problems are that do not work too much. Finish the job almost, then use some kind of absorbing configuration (that is what we call laziness).”
Montgomery, Pokrovskiy, and Sudakov’s paper concludes with two related famous conjectures:
The graceful tree conjecture
Motivated by Ringel’s conjecture, in 1967 Alexander Rosa conjectured that every tree T with n edges admits a graceful labeling, namely a labeling of the vertices by the numbers 0,1,…,n such that if we label an edge by the absolute value of the difference of the labels for its two vertices, then all the edge labels are different!
This famous conjecture (stronger than Ringel’s conjecture) is known as the Graceful Tree conjecture and also as the Ringel-Rosa-Kötzig conjecture. It is still open.
Packing trees of different sizes: Gyárfás conjecture
Gyárfás conjecture (1976): Let be trees with having vertices for each . Then the edges of can be decomposed into trees which are isomorphic to respectively.