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A019309
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Number of "bifix-free" words of length n over a four-letter alphabet.
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8
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1, 4, 12, 48, 180, 720, 2832, 11328, 45132, 180528, 721392, 2885568, 11539440, 46157760, 184619712, 738478848, 2953870260, 11815481040, 47261743632, 189046974528, 756187176720, 3024748706880, 12098991941952
(list;
graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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0,2
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LINKS
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FORMULA
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a(2n+1) = 4a(2n); a(2n) = 4a(2n-1) - a(n).
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MATHEMATICA
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a[0]=1; a[n_]:=a[n]=4*a[n-1]-If[EvenQ[n], a[n/2], 0] (* Ed Pegg Jr, Jan 05 2005 *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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