The eigenfunctions, calculated using the expansion method, corresponding to the most unstable eigenvalues σ= 0.875505 ı, 3.12112 ı and 3.70786 ı for k= 10 (full curve), 100 (long-dashed) and 500 (dashed), respectively, in the symmetric case for the banded profile of Fig. 2. The 5° magnetic band is centred on μ= 0.5, indicated by the vertical line. N= 1 throughout.