OFFSET
1,1
COMMENTS
Note that for n > 6, we need examine only squarefree n having three or more factors. Sequence A095878 has the values of the middle coefficient of Phi_n(x) for the n in this sequence. Note that for n = 1 and 2, Phi_n(x) is a polynomial having no middle term.
LINKS
G. Dresden, On the Middle Coefficient of a Cyclotomic Polynomial (MAA Monthly, 2004, 531 - 533).
EXAMPLE
385 is in this sequence because the middle coefficient of Phi_385(x) is -3, which does not occur for any n < 385.
MATHEMATICA
vals={1, 0, -1}; lst={3, 4, 6}; Do[pwrs=Transpose[FactorInteger[n]][[2]]; If[Length[pwrs]>2 && 1==Times@@pwrs, c=Coefficient[Cyclotomic[n, x], x, EulerPhi[n]/2]; If[ !MemberQ[vals, c], AppendTo[lst, n]; AppendTo[vals, c]]], {n, 7, 50000, 2}]; lst
CROSSREFS
KEYWORD
hard,nonn
AUTHOR
T. D. Noe, Jun 10 2004
EXTENSIONS
More terms from Jozsef Pelikan, Dec 18 2014
STATUS
approved