A tree not having the complete bipartite graph with base at the vertex of degree two as a limb (Lu et al. 1993, Lu 1996).
M-Tree
See also
TreeExplore with Wolfram|Alpha
References
Lu, T. "The Enumeration of Trees with and without Given Limbs." Disc. Math. 154, 153-165, 1996.Lu, T. J.; Read, R. C.; and Palmer, E. M. "On the Enumeration of Trees with Certain Local Restrictions." Congr. Numer. 95, 183-202, 1993.Referenced on Wolfram|Alpha
M-TreeCite this as:
Weisstein, Eric W. "M-Tree." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/M-Tree.html