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%I #16 Nov 09 2021 02:57:02
%S 431985,757335,865365,1134915,1296885,1297815,1675365,1729335,1891815,
%T 2161785,2162715,2595165,2648715,2649585,3027165,3028035,3132015,
%U 3133785,3347985,3405615,3565785,3784065,3891585,4698465,4920285,5188935,5189865,5676315
%N Numbers n such that Phi(n,x) is a flat cyclotomic polynomial of order four.
%C A flat polynomial is defined to be a polynomial whose coefficients are -1, 0, or 1. Order four means that n is the product of four odd primes p<q<r<s.
%C For pqrs to be flat, it appears that three conditions on p < q < r < s are required: q = -1 (mod p), r = +-1 (mod pq), and s = +-1 (mod pqr). [_T. D. Noe_, Apr 13 2010]
%H T. D. Noe, <a href="/A117318/b117318.txt">Table of n, a(n) for n=1..216</a>
%H Nathan Kaplan, <a href="http://www.emis.de/journals/INTEGERS/papers/k30/k30.Abstract.html">Flat cyclotomic polynomials of order four and higher</a>, #A30, Integers 10 (2010), 357-363.
%H Carlo Sanna, <a href="https://arxiv.org/abs/2111.04034">A Survey on Coefficients of Cyclotomic Polynomials</a>, arXiv:2111.04034 [math.NT], 2021.
%Y Cf. A117223 (third-order flat cyclotomic polynomials).
%K nonn
%O 1,1
%A _T. D. Noe_, Mar 07 2006
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