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A118729
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Rectangular array where row r contains the 8 numbers 4*r^2 - 3*r, 4*r^2 - 2*r, ..., 4*r^2 + 4*r.
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14
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0, 0, 0, 0, 0, 0, 0, 0, 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 14, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 42, 45, 48, 52, 56, 60, 64, 68, 72, 76, 80, 85, 90, 95, 100, 105, 110, 115, 120, 126, 132, 138, 144, 150, 156, 162, 168
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OFFSET
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0,10
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COMMENTS
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The numbers in row r span the interval ]8*A000217(r-1), 8*A000217(r)].
The first difference between the entries in row r is r.
a(n+7) is the number of key presses required to type a word of n letters, all different, on a keypad with 8 keys where 1 press of a key is some letter, 2 presses is some other letter, etc., and under an optimal mapping of letters to keys and presses (answering LeetCode problem 3014). - Christopher J. Thomas, Feb 16 2024
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LINKS
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Index entries for linear recurrences with constant coefficients, signature (2,-1,0,0,0,0,0,1,-2,1).
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FORMULA
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EXAMPLE
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The array starts, with row r=0, as
r=0: 0 0 0 0 0 0 0 0;
r=1: 1 2 3 4 5 6 7 8;
r=2: 10 12 14 16 18 20 22 24;
r=3: 27 30 33 36 39 42 45 48;
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MATHEMATICA
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Flatten[Table[4r^2+r(Range[-3, 4]), {r, 0, 6}]] (* or *) LinearRecurrence[ {2, -1, 0, 0, 0, 0, 0, 1, -2, 1}, {0, 0, 0, 0, 0, 0, 0, 0, 1, 2}, 60] (* Harvey P. Dale, Nov 26 2015 *)
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CROSSREFS
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KEYWORD
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nonn,tabf,easy
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AUTHOR
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Stuart M. Ellerstein (ellerstein(AT)aol.com), May 21 2006
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EXTENSIONS
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Redefined as a rectangular tabf array and description simplified by R. J. Mathar, Oct 20 2010
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STATUS
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approved
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