EN|RU Volume 17, No 5, 2010, P. 67-90 UDC 519.174 L. A. Sholomov A rule for the addition of entropies for underdetermined data Abstract: The basic information characteristics for underdetermined data are introduced. For underdetermined data of the general form, it is proved that the entropy addition rule $H(X) + H(Y |X) = H(XY)$ of classic Information theory turns into a more complicated expression. The form of the expression is found. In addition, the necessary and sufficient conditions for it to coincide with the ordinary entropy addition rule are obtained. Bibliogr. 11. Keywords: underdetermined data, information characteristics, entropy, conditional entropy, specification, best specification, entropy addition rule. Sholomov Lev Abramovich 1 1.Institute of System Analysis RAS, 9, 60-letiya Oktyabrya ave., 117312 Moscow, Russia e-mail: sholomov@isa.ru © Sobolev Institute of Mathematics, 2015