Markov chain beam randomization: a study of the impact of PLANCK beam measurement errors on cosmological parameter estimationG. Rocha, L. Pagano, K. M. Górski, K. M. Huffenberger, C. R. Lawrence and A. E. LangeA&A, 513 (2010) A23DOI: https://doi.org/10.1051/0004-6361/200913032