MathDB
Ascent and descent

Source: Indian IMOTC 2005 Day 3 problem 3

September 23, 2005
analytic geometrycombinatorics unsolvedcombinatorics

Problem Statement

A merida path of order 2n2n is a lattice path in the first quadrant of xyxy- plane joining (0,0)(0,0) to (2n,0)(2n,0) using three kinds of steps U=(1,1)U=(1,1), D=(1,1)D= (1,-1) and L=(2,0)L= (2,0), i.e. UU joins x,y)x,y) to (x+1,y+1)(x+1,y+1) etc... An ascent in a merida path is a maximal string of consecutive steps of the form UU. If S(n,k)S(n,k) denotes the number of merdia paths of order 2n2n with exactly kk ascents, compute S(n,1)S(n,1) and S(n,n1)S(n,n-1).