What is the fair market value of an object that cannot be sold?

The question may sound like a Zen koan, but it is one that lawyers for the heirs of the New York art dealer Ileana Sonnabend and the Internal Revenue Service are set to debate when they meet in Washington next month.

The object under discussion is “Canyon,” a masterwork of 20th-century art created by Robert Rauschenberg that Mrs. Sonnabend’s children inherited when she died in 2007.

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