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A276544
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Triangle read by rows: T(n,k) = number of primitive (aperiodic) reversible string structures with n beads using exactly k different colors.
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10
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1, 0, 1, 0, 2, 1, 0, 4, 4, 1, 0, 9, 15, 6, 1, 0, 16, 49, 37, 9, 1, 0, 35, 160, 183, 76, 12, 1, 0, 66, 498, 876, 542, 142, 16, 1, 0, 133, 1544, 3930, 3523, 1346, 242, 20, 1, 0, 261, 4715, 17179, 21392, 11511, 2980, 390, 25, 1
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OFFSET
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1,5
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COMMENTS
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A string and its reverse are considered to be equivalent. Permuting the colors will not change the structure.
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REFERENCES
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M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2]
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LINKS
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FORMULA
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T(n, k) = Sum_{d|n} mu(n/d) * A284949(d, k).
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EXAMPLE
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Triangle starts
1
0 1
0 2 1
0 4 4 1
0 9 15 6 1
0 16 49 37 9 1
0 35 160 183 76 12 1
0 66 498 876 542 142 16 1
0 133 1544 3930 3523 1346 242 20 1
0 261 4715 17179 21392 11511 2980 390 25 1
...
Primitive reversible word structures are:
n=1: a => 1
n=2: ab => 1
n=3: aab, aba; abc => 2 + 1
n=4: aaab, aaba, aabb, abba => 4 (k=2)
aabc, abac, abbc, abca => 4 (k=3)
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MATHEMATICA
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Ach[n_, k_] := Ach[n, k] = Switch[k, 0, If[n == 0, 1, 0], 1, If[n > 0, 1, 0], _, If[OddQ[n], Sum[Binomial[(n - 1)/2, i] Ach[n - 1 - 2 i, k - 1], {i, 0, (n - 1)/2}], Sum[Binomial[n/2 - 1, i] (Ach[n - 2 - 2 i, k - 1] + 2^i Ach[n - 2 - 2 i, k - 2]), {i, 0, n/2 - 1}]]]
T[n_, k_] := DivisorSum[n, MoebiusMu[n/#] (StirlingS2[#, k] + Ach[#, k])/2& ];
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PROG
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(PARI) \\ here Ach is A304972 as matrix.
Ach(n, m=n)={my(M=matrix(n, m, i, k, i>=k)); for(i=3, n, for(k=2, m, M[i, k]=k*M[i-2, k] + M[i-2, k-1] + if(k>2, M[i-2, k-2]))); M}
T(n, m=n)={my(M=matrix(n, m, i, k, stirling(i, k, 2)) + Ach(n, m)); matrix(n, m, i, k, sumdiv(i, d, moebius(i/d)*M[d, k]))/2}
{ my(A=T(10)); for(n=1, #A, print(A[n, 1..n])) } \\ Andrew Howroyd, Jan 09 2020
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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