@article{ahmadi:1985:ult,
    author = {Ahmadi, A. R. and Widnall, S. E.},
     title = {Unsteady lifting-line theory as a singular-perturbation
              problem},
   journal = {J. Fluid Mech},
    volume = {153},
      year = {1985},
     pages = {59–81},
       doi = {10.1017/S0022112085001148},
     zblno = {0589.76027}}