Packing the largest trees in the tree packing conjecture

Abstract

The famous tree packing conjecture of Gyárfás from 1976 says that any sequence of trees $T_1,\dots ,T_n$ such that $|T_i|=i$ for each $i\in [n]$ packs into the complete $n$-vertex graph $K_n$. Packing even just the largest trees in such a sequence has proven difficult, with Bollobás drawing attention to this in 1995 by conjecturing that, for each $k$, if $n$ is sufficiently large then the largest $k$ trees in any such sequence can be packed into $K_n$. This has only been shown for $k\le 5$, by Żak, despite many partial results and much related work on the full tree packing conjecture. We prove Bollobás’s conjecture, by showing that, moreover, a linear number of the largest trees can be packed in the tree packing conjecture.