Journal of Integer Sequences, Vol. 28 (2025), Article 25.1.6

Counting Subwords in Non-Decreasing Dyck Paths


Rigoberto Flórez
Department of Mathematical Sciences
The Citadel
Charleston, SC 29409
USA

Leandro Junes
Department of Mathematics
PennWest California
California, PA 15419
USA

Luisa M. Montoya and José L. Ramírez
Departamento de Matemáticas
Universidad Nacional de Colombia
Bogotá
Colombia

Abstract:

A Dyck path is non-decreasing if the y-coordinates of its valleys, which are local minima along the path, form a non-decreasing sequence. To count the number of subpaths (or subwords) within non-decreasing Dyck paths, we use generating functions and recursive relations. These techniques enable us to count subpaths of various lengths, such as two, three, four, and five, while considering both the length and the number of subwords. Using constructive recursive proofs, we establish a combinatorial interpretation of the counting process, expressing the results as linear combinations of Fibonacci and Lucas numbers. In several cases, we provide new interpretations of sequences from the On-Line Encyclopedia of Integer Sequences (OEIS).


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(Concerned with sequences A000108 A001870 A001871 A002212 A030267 A033321 A038731 A039598 A054444 A059502 A105306 A105693 A121463 A121466 A122737 A153342 A197649 A238846 A304429 A375995 A377670 A377679 A377857 A377866 A377867 A378383.)


Received November 5 2024; revised version received February 12 2025. Published in Journal of Integer Sequences, February 13 2025.


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