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A102190
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Irregular triangle read by rows: coefficients of cycle index polynomial for the cyclic group C_n, Z(C_n,x), multiplied by n.
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21
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1, 1, 1, 1, 2, 1, 1, 2, 1, 4, 1, 1, 2, 2, 1, 6, 1, 1, 2, 4, 1, 2, 6, 1, 1, 4, 4, 1, 10, 1, 1, 2, 2, 2, 4, 1, 12, 1, 1, 6, 6, 1, 2, 4, 8, 1, 1, 2, 4, 8, 1, 16, 1, 1, 2, 2, 6, 6, 1, 18, 1, 1, 2, 4, 4, 8, 1, 2, 6, 12, 1, 1, 10, 10, 1, 22, 1, 1, 2, 2, 2, 4, 4, 8, 1, 4, 20, 1, 1, 12, 12, 1, 2, 6, 18, 1, 1, 2, 6, 6
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OFFSET
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1,5
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COMMENTS
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Row n gives the coefficients of x[k]^{n/k} with increasing divisors k of n.
The length of row n is tau(n) = A000005(n) (number of divisors of n, including 1 and n).
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REFERENCES
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N. G. De Bruijn, Polya's theory of counting, in E. F. Beckenbach, ed., Applied Combinatorial Mathematics, Wiley, 1964, pp. 144-184 (see Example 5.7).
F. Harary, Graph Theory, Addison-Wesley, Reading, MA, 1994; pp. 181 and 184.
F. Harary and E. M. Palmer, Graphical Enumeration, Academic Press, NY, 1973, p. 36, (2.2.10).
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LINKS
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FORMULA
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a(n, m) = phi(k(m)), m=1..tau(n), n>=1, with k(m) the m-th divisor of n, written in increasing order.
Z(C_n, x):=sum(sum(phi(k)*x[k]^{n/k}, k|n))/n, where phi(n)= A000010(n) (Euler's totient function) and k|n means 'k divides n'. Cf. Harary-Palmer reference and MathWorld link.
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EXAMPLE
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Array begins:
1: [1],
2: [1, 1],
3: [1, 2],
4: [1, 1, 2],
5: [1, 4],
6: [1, 1, 2, 2],
7: [1, 6], ...
The entry for n=6 is obtained as follows:
Z(C_6,x)=(1*x[1]^6 + 1*x[2]^3 + 2*x[3]^2 + 2*x[6]^1)/6.
a(6,1)=phi(1)=1, a(6,2)=phi(2)=1, a(6,3)=phi(3)=2, a(6,4)=phi(6)=2.
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MATHEMATICA
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k[n_, m_] := Divisors[n][[m]]; a[n_, m_] := EulerPhi[k[n, m]]; Flatten[Table[a[n, m], {n, 1, 28}, {m, 1, DivisorSigma[0, n]}]] (* Jean-François Alcover, Jul 25 2011, after given formula *)
row[n_] := If[n == 1, {1}, n List @@ CycleIndexPolynomial[CyclicGroup[n], Array[x, n]] /. x[_] -> 1]; Array[row, 30] // Flatten (* Jean-François Alcover, Nov 04 2016 *)
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PROG
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(PARI) tabf(nn) = for (n=1, nn, print(apply(x->eulerphi(x), divisors(n)))); \\ Michel Marcus, Nov 13 2015
(PARI) tabf(nn) = for (n=1, nn, print(apply(x->poldegree(x), factor(x^n-1)[, 1]))) \\ Michel Marcus, Nov 13 2015
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CROSSREFS
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KEYWORD
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nonn,easy,tabf
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AUTHOR
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STATUS
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approved
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