#890 · Combinatorics · Counting
Telegraph signals
A telegraph has five arms, each of which can be placed in one of four distinct positions (including the rest position). How many different signals can be produced, excluding the signal in which all arms are at rest?
H. S. Hall & S. R. Knight, Higher Algebra (1891) · Ch. XI. Permutations and Combinations, Examples XI.b, No. 17
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