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A027428
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Number of distinct products ij with 1 <= i < j <= n.
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6
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0, 1, 3, 6, 10, 13, 19, 24, 31, 36, 46, 51, 63, 70, 78, 87, 103, 111, 129, 139, 150, 161, 183, 192, 210, 223, 239, 252, 280, 291, 321, 337, 354, 371, 390, 403, 439, 458, 478, 493, 533, 549, 591, 611, 631, 654, 700, 717, 752, 774, 800, 823, 875
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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1,3
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LINKS
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T. D. Noe, Table of n, a(n) for n=1..1000
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FORMULA
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a(n)=A027427(n)-1. - T. D. Noe, Jan 16 2007
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MAPLE
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f:=proc(n) local i, j, t1, t2; t1:={}; for i from 1 to n-1 do for j from i+1 to n do t1:={op(t1), i*j}; od: od: t1:=convert(t1, list); nops(t1); end;
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MATHEMATICA
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a[n_] := Table[i*j, {i, 1, n-1}, {j, i+1, n}] // Flatten // Union // Length; Table[ a[n] , {n, 1, 53}] (* _Jean-François Alcover, Jan 31 2013 *)
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PROG
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(Haskell)
import Data.List (nub)
a027428 n = length $ nub [i*j | j <- [2..n], i <- [1..j-1]]
-- Reinhard Zumkeller, Jan 01 2012
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CROSSREFS
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Cf. A027424, A027430.
Sequence in context: A049880 A189368 A128039 * A136850 A079248 A083505
Adjacent sequences: A027425 A027426 A027427 * A027429 A027430 A027431
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KEYWORD
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nonn,easy,nice
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AUTHOR
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N. J. A. Sloane.
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STATUS
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approved
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