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A179804
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Number of letter combinations on a standard telephone keypad represented by the digits in n
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0
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0, 3, 3, 3, 3, 3, 3, 3, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0, 0, 9, 9, 9, 9, 9, 9, 9, 9, 0
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OFFSET
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1,2
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LINKS
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FORMULA
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If n contains a 1 or 0, then a(n) = 0
Otherwise, if d=number of digits in n, then a(n)=3^d.
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EXAMPLE
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If any digit in n is 1 or 0, then a(n) = 0 because telephone keys 1 and 0 represent no letters
a(2)=3 because "3" on a keypad can represent "d", "e", or "f".
a(23)=9 because "2" on a keypad represents the letters "abc" and "3" represents "def", and there are 9 possible combinations taking one letter from each of these sets in this order.
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CROSSREFS
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KEYWORD
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easy,nonn,base
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AUTHOR
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STATUS
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approved
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