Past exam of the mathematics course of the University of Cambridge 2014 iii Paper 47 2 ii Solution Created 2026-10-03 Updated 2026-10-06
For , choose the positive Bogomolny equation and constant angular orientation . Integrating givesThis interpolates from to , so . Its useful profiles are and . It obeys the second-order field equation because differentiating gives ; the angular field equation holds since is constant. ThusBoth and are collective coordinates: translation and global angular rotation do not change the energy. This static solution is the zero-charge member of the rotating charged kink in a spherical sigma model.
Past exam of the mathematics course of the University of Cambridge 2014 iii Paper 47 2 iv Solution Created 2026-10-03 Updated 2026-10-06
At , the field equations areWith and static , the second equation holds automatically and the first reduces toFor , put . The finite-energy solution isThis is a rotating charged kink in a spherical sigma model. Its orientation moves around a circle while its energy profile stays at rest. The limiting value gives no localized finite-width kink; larger does not give this finite-energy interpolation.