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A026541
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a(n) = T(n,n-4), T given by A026536. Also a(n) = number of integer strings s(0), ..., s(n), counted by T, such that s(n) = 4.
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2
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1, 2, 9, 19, 65, 136, 430, 886, 2721, 5538, 16793, 33887, 102102, 204856, 615024, 1229280, 3682545, 7341786, 21963161, 43712603, 130648089, 259726104, 775797750, 1541084142, 4601346295, 9135694750, 27270124455, 54125522793
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OFFSET
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4,2
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LINKS
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FORMULA
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MATHEMATICA
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T[n_, k_]:= T[n, k]= If[k==0 || k==2*n, 1, If[k==1 || k==2*n-1, Floor[n/2], If[EvenQ[n], T[n-1, k-2] +T[n-1, k-1] +T[n-1, k], T[n-1, k-2] +T[n-1, k]] ]];
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PROG
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(SageMath)
@CachedFunction
if k < 0 or n < 0: return 0
elif k == 0 or k == 2*n: return 1
elif k == 1 or k == 2*n-1: return n//2
elif n % 2 == 1: return T(n-1, k-2) + T(n-1, k)
return T(n-1, k-2) + T(n-1, k-1) + T(n-1, k)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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