Yrast line for weakly interacting trapped bosons

We compute numerically the yrast line for harmonically trapped N-boson syst ems with a weak repulsive contact interaction, studying the transition to a vortex state as the angular momentum L increases and approaches N. There i s a smooth crossover between low and high L. with no indication of a quantu m phase transition; at L = N there is a kink in the slope of the spectrum. Most strikingly, the energy and wave function appear to be analytical funct ions of L over the entire range 2 less than or equal to L less than or equa l to N. We confirm the structure of low-L states proposed by Mottelson as m ainly single-particle excitations with two or three units of angular moment um.