Abstract
S. M. Ulam, (“Adventures of a Mathematician,” Scribner's, 1976.) stated the following problem: what is the minimal number of yes-no queries needed to find an integer between one and one million, if one lie is allowed among the answers. In Rivest et al. ( J. Comput. System Sci 20, 396–404 (1980) and Spencer, ( Math. Mag. 57, 105–108 (1984) partial solutions were given by establishing bounds for the minimal number of queries necessary to find a number in the set {1,…, n}. Applied to the original question both solutions yield two possibilities: 25 or 26. We give an exact solution of Ulam's problem in the general case. For n = 10 6 the answer turns out to be 25. We also give an algorithm to perform the search using the minimal number of queries.
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