Abstract
Chakrabarti and Manna (1983) have made a numerical study of 'directed' self-avoiding walks on a square lattice, in which the random walker is forbidden to move upward: extrapolating from an enumeration of N-step walks up to N=14, they claimed that the mean end-to-end displacement, RN, behaves asymptotically for large N as Nnu with nu =0.86+or-0.02. It is shown that a simple application of the generating function method yields the behaviour of N-step walks of this type exactly (for all N), and the result nu =1 is rigorously proved.