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Search: id:A130319
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| A130319 |
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Number n for which n!!/S(n) is integer, where S(n) is the sum of all odd numbers minor or equal to n, if n is odd, or the sum of all even numbers minor or equal to n, if n is even. |
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+0
3
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| 1, 2, 6, 10, 14, 16, 17, 18, 22, 26, 28, 29, 30, 34, 38, 40, 41, 42, 46, 48, 49, 50, 52, 53, 54, 58, 62, 64, 65, 66, 68, 69, 70, 74, 76, 77, 78, 82, 86, 88, 89, 90, 94, 96, 97, 98, 100
(list; graph; listen)
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OFFSET
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0,2
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EXAMPLE
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6 --> 6!! = 48; 6 + 4 + 2 = 12; 48/12 = 4.
17 --> 17!! = 34459425; 17 + 15 + 13 + 11 + 9 + 7 + 5 + 3 + 1 = 81; 34459425/81 = 425425.
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MAPLE
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P:=proc(n) local a, i, j, k, w; for i from 1 by 1 to n do k:=i; w:=i-2; while w>0 do k:=k*w; w:=w-2; od; j:=i; w:=i-2; while w>0 do j:=j+w; w:=w-2; od; a:=k/j; if trunc(a)=a then print(i) fi; od; end: P(100);
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CROSSREFS
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Cf. A108552, A000290, A005408, A130318.
Adjacent sequences: A130316 A130317 A130318 this_sequence A130320 A130321 A130322
Sequence in context: A074929 A095270 A080324 this_sequence A067368 A080456 A068977
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KEYWORD
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easy,nonn
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AUTHOR
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Paolo P. Lava & Giorgio Balzarotti (ppl(AT)spl.at), May 23 2007
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