@article{mulholland:1929:me,
    author = {Mulholland, H. P. and Goldstein, S.},
     title = {The characteristic numbers of the {M}athieu equation with purely
              imaginary parameter},
   journal = {Phil. Mag. Series 7},
    volume = {8},
    number = {53},
      year = {1929},
     pages = {834–840},
       doi = {10.1080/14786441208564946},
     zblno = {55.0261.01}}