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A027057
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a(n) = (1/2) * A027052(n, 2n-1).
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2
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1, 2, 4, 9, 21, 51, 128, 329, 861, 2285, 6132, 16606, 45313, 124446, 343680, 953753, 2658133, 7436541, 20875972, 58783892, 165989825, 469903672, 1333359488, 3791535934, 10802911297, 30836181436, 88169413364, 252500533673, 724182805389, 2079862921763, 5981150599872
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graph;
refs;
listen;
history;
text;
internal format)
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OFFSET
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2,2
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LINKS
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MAPLE
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T:= proc(n, k) option remember;
if k<0 or k>2*n then 0
elif k=0 or k=2 or k=2*n then 1
elif k=1 then 0
else add(T(n-1, k-j), j=1..3)
fi
end:
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MATHEMATICA
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T[n_, k_]:= T[n, k]= If[k<0 || k>2*n, 0, If[k==0 || k==2 || k==2*n, 1, If[k==1, 0, Sum[T[n-1, k-j], {j, 3}]]]]; Table[T[n, 2*n-1]/2, {n, 2, 30}] (* G. C. Greubel, Nov 06 2019 *)
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PROG
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(Sage)
@CachedFunction
def T(n, k):
if (k<0 or k>2*n): return 0
elif (k==0 or k==2 or k==2*n): return 1
elif (k==1): return 0
else: return sum(T(n-1, k-j) for j in (1..3))
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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