Magic Labeling
It is conjectured that every Tree with 
edges whose nodes are all trivalent or monovalent can be given a ``magic''labeling such that the Integers 1, 2, ..., can be assigned to the edges so that the Sum of thethree meeting at a node is constant.
References
Guy, R. K. ``Unsolved Problems Come of Age.'' Amer. Math. Monthly 96, 903-909, 1989.
- Markoff Number
- Markoff's Formulas
- Markov Algorithm
- Markov Chain
- Markov Matrix
- Markov Moves
- Markov Number
- Markov Process
- Markov's Inequality
- Markov Spectrum
- Markov's Theorem
- Marriage Theorem
- Married Couples Problem
- Marshall-Edgeworth Index
- Martingale
- Mascheroni Constant
- Mascheroni Construction
- Maschke's Theorem
- Mason's abc Theorem
- Mason's Theorem
- Masser-Gramain Constant
- Match Problem
- Matchstick Graph
- Mathematical Induction
- Mathematics