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A344673
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Numbers k such that the expansion of the inverse of the k-th cyclotomic polynomial has a coefficient other than -1, 0 or 1.
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2
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561, 595, 665, 741, 935, 1001, 1105, 1122, 1155, 1173, 1190, 1309, 1330, 1365, 1463, 1479, 1482, 1495, 1615, 1683, 1729, 1767, 1785, 1870, 1955, 1995, 2001, 2002, 2015, 2093, 2145, 2185, 2210, 2223, 2233, 2244, 2261, 2310, 2346, 2380, 2387, 2415, 2431, 2465, 2618
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OFFSET
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1,1
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COMMENTS
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Define Psi_n(x) = (x^n-1)/Phi_n(x), with Phi_n(x) the n-th cyclotomic polynomial, then 1/Phi_n(x) = -Psi_n(x) * (1 + x^n + x^(2n) + ...), hence the period of coefficients in the expansion of 1/Phi_n(x) is n.
For odd k, k is a term if and only 2*k is a term.
For prime p dividing k, k is a term if and only if p*k is a term.
The period of the expansion of Psi(n) is A062830(n).
Terms are neither prime nor semiprime.
Least k having a run of j consecutive terms, for j >= 0: 561, 2001, 22630, 68263, ...
(End)
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LINKS
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EXAMPLE
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1/Phi_561(x) = 1 - x + x^3 - x^4 + x^6 - x^7 + x^9 - x^10 + x^11 - x^13 + x^14 - x^16 + 2*x^17 + ..., the coefficient of x^17 is 2, so 561 is a term.
1/Phi_595(x) = 1 - x + x^5 - x^6 + x^7 - x^8 + x^10 - x^11 + x^12 - x^13 + x^14 - x^16 + 2*x^17 + ..., the coefficient of x^17 is 2, so 595 is a term.
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MATHEMATICA
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fQ[n_] := Max@ Union@ Abs@ CoefficientList[ Simplify[(x^n - 1)/Cyclotomic[n, x]], x] > 1; Select[ Range@ 2650, fQ] (* Robert G. Wilson v, May 26 2021 *)
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PROG
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(PARI) isA344673(k) = (vecmax(abs(Vec((x^k-1)/polcyclo(k))))>=2)
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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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