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A117621
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Number of double-perfect partitions of [1..n].
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1
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0, 1, 1, 1, 1, 1, 3, 1, 3, 2, 3, 1, 7, 1, 3, 3, 6, 1, 8, 1, 7, 3, 3, 1, 17, 2, 3, 4, 7, 1, 13, 1, 12, 3, 3, 3, 24, 1, 3, 3, 17, 1, 13, 1, 7, 8, 3, 1, 40, 2, 8, 3, 7, 1, 20, 3, 17, 3, 3, 1, 41, 1, 3, 8, 24, 3, 13, 1, 7, 3, 13, 1, 68, 1, 3, 8, 7, 3, 13, 1, 40, 8, 3, 1, 41, 3, 3, 3, 17, 1, 44, 3, 7, 3, 3, 3
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,7
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REFERENCES
| HoKyu Lee, Double perfect partitions, Discrete Math., 306 (2006), 519-525.
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FORMULA
| a(1)=0; a(n)=1 for n=2..5; a(n) = Sum_{m=2..n-1, m-1|n-1} a(m) for n >= 6.
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MAPLE
| f:=proc(n) option remember; local t1, m, nm1, mm1; nm1:=n-1; if n <= 1 then RETURN(0); elif n <= 5 then RETURN(1); else t1:=0; for m from 2 to n-1 do mm1:=m-1; if nm1 mod mm1 = 0 then t1:=t1+f(m); fi; od; RETURN(t1); fi; end;
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CROSSREFS
| Cf. A002033.
Sequence in context: A055189 A106824 A123508 * A178055 A059660 A194003
Adjacent sequences: A117618 A117619 A117620 * A117622 A117623 A117624
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KEYWORD
| nonn
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Apr 07 2006
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