Andrej Muchnik Alexander Shen Mikhail Ustinov Nikolai K. Vereshchagin Michael V. Vyugin Non-reducible descriptions for conditional Kolmogorov complexity. 77-86 2007 384 Theor. Comput. Sci. 1 http://dx.doi.org/10.1016/j.tcs.2007.05.019 db/journals/tcs/tcs384.html#MuchnikSUVV07