  We compute numerically the yrast line for harmonically trapped boson systems
with a weak repulsive contact interaction, studying the transition to a vortex
state as the angular momentum L increases and approaches N, the number of
bosons. The L=N eigenstate is indeed dominated by particles with unit angular
momentum, but the state has other significant components beyond the pure vortex
configuration. There is a smooth crossover between low and high L with no
indication of a quantum phase transition. Most strikingly, the energy and wave
function appear to be analytical functions of L over the entire range 2 < L <
N. We confirm the structure of low-L states proposed by Mottelson, as mainly
single-particle excitations with two or three units of angular momentum.
