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D. L. MacKay, Paul D. Thomas, Walter B. Clarke, Arnold Kramish, W. V. Parker, J. M. Hurt, V. Thebault, William N. Huff, C. E. Springer, E. P. Starke, Problem Department, National Mathematics Magazine, Vol. 16, No. 6 (Mar., 1942), pp. 305-310
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MacKay et al. (1942) studied this question.
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