In 1991 Gnanajothi conjectured: Each tree is odd-graceful. In this paper, we define the edge-ordered odd-graceful labeling of trees and show the odd-gracefulness of all symmetric trees.
Xianggian ZHou, Bing YAo, Ming YAo®, Xiang’en CHEN. On Odd-gracefulness of All Symmetric Trees[J], Journal of Combinatorial Mathematics and Combinatorial Computing, Volume 087. 309-319. DOI: .