Published in Proceedings of the IRE.
Cited passages: David A. Huffman, A Method for the Construction of Minimum-Redundancy Codes, p. 1099, Derived Coding Requirements: equal-probability messages may have unequal lengths and may exchange codes without changing the mean; David A. Huffman, A Method for the Construction of Minimum-Redundancy Codes, p. 1099: Derived Coding Requirements and Optimum Binary Code.