Mireille Bousquet-Mélou ; Yann Ponty - Culminating paths

dmtcs:438 - Discrete Mathematics & Theoretical Computer Science, June 5, 2007, Vol. 10 no. 2 - https://doi.org/10.46298/dmtcs.438
Culminating paths

Authors: Mireille Bousquet-Mélou ; Yann Ponty

Let a and b be two positive integers. A culminating path is a path of Z^2 that starts from (0,0), consists of steps (1,a) and (1,-b), stays above the x-axis and ends at the highest ordinate it ever reaches. These paths were first encountered in bioinformatics, in the analysis of similarity search algorithms. They are also related to certain models of Lorentzian gravity in theoretical physics. We first show that the language on a two letter alphabet that naturally encodes culminating paths is not context-free. Then, we focus on the enumeration of culminating paths. A step by step approach, combined with the kernel method, provides a closed form expression for the generating fucntion of culminating paths ending at a (generic) height k. In the case a=b, we derive from this expression the asymptotic behaviour of the number of culminating paths of length n. When a>b, we obtain the asymptotic behaviour by a simpler argument. When a= b, with no precomputation stage nor non-linear storage required. The choice of the best algorithm is not as clear when a


Volume: Vol. 10 no. 2
Published on: June 5, 2007
Imported on: March 26, 2015
Keywords: Lattice paths,enumeration,random generation,05A15, 05A16 , 60C05,[MATH.MATH-CO] Mathematics [math]/Combinatorics [math.CO]


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