@ARTICLE{g2015a,
author = {D. Gallistl},
title = {An optimal adaptive {FEM} for eigenvalue clusters},
journal = {Numer. Math.},
year = {2015},
doi = {10.1007/s00211-014-0671-8},
fjournal = {Numerische Mathematik},
volume = {130},
number = {3},
pages = {467--496},
}