(:notitlegroup:) 

   @article{cockrell2011jasa,
    title     = {A Modal Wentzel-Kramers-Brillouin Approach to Calculating the Waveguide
                Invariant for Non-ideal Waveguides},
    author    = {Kevin L. Cockrell and Henrik Schmidt},
    journal   = {The Journal of the Acoustical Society of America},
    pages     = {72-83},
    number    = {1},
    volume    = {130},
    month     = {Jyly},
    year      = {2011}}