On the number of Abelian square-free words on four letters

Some unsolved problems.

Paul Erdős · The Michigan Mathematical Journal · 1957 · 336 citations · Full text

Natural Sciences, Mathematics, Geometry +6

Avoidable patterns in strings of symbols

Dwight Bean, Andrzej Ehrenfeucht, George F. McNulty · Pacific Journal of Mathematics · 1979 · 308 citations · Full text

Natural Sciences, Mathematics, Combinatorics +5

Uniformly growing k-th power-free homomorphisms

Franz–Josef Brandenburg · Theoretical Computer Science · 1988 · 152 citations

Natural Sciences, High-performance Computing, Combinatorics +3

Non-repetitive sequences

P. A. B. Pleasants · Mathematical Proceedings of the Cambridge Philosophical Society · 1970 · 143 citations

Natural Sciences, Mathematics, Sequence Analysis +3

Strongly non-repetitive sequences and progression-free sets

F. M. Dekking · Journal of Combinatorial Theory Series A · 1979 · 136 citations

Natural Sciences, Mathematics, Combinatorics +3