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A123908
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Number of sequences with terms 1, 2 or 3 summing to n with no three consecutive 1's.
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0
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1, 1, 2, 3, 6, 10, 17, 30, 52, 90, 156, 271, 470, 815, 1414, 2453, 4255, 7381, 12804, 22211, 38529, 66836, 115940, 201120, 348881, 605201, 1049837, 1821143, 3159121, 5480100, 9506282, 16490465, 28605867, 49622350, 86079461, 149321296
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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FORMULA
| a(n) = a(n - 2) + 2a(n - 3) + 2a(n - 4) + a(n - 5)
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EXAMPLE
| a(4) = 6 as 3 + 1, 1 + 3, 2 + 2, 1 + 1 + 2, 1 + 2 + 1 and 2 + 1 + 1 (but not 1 + 1 + 1 + 1).
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MAPLE
| a[0]=a[1]=1 a[2]=2 a[3]=3 a[4]=6 a[n_] := a[n - 2] + 2a[n - 3] + 2a[n - 4] + a[n - 5]
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CROSSREFS
| Sequence in context: A023614 A001610 A135431 * A026397 A193200 A026779
Adjacent sequences: A123905 A123906 A123907 * A123909 A123910 A123911
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KEYWORD
| easy,nonn
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AUTHOR
| Joel Lewis (jblewis(AT)fas.harvard.edu), Oct 28 2006
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