OFFSET
1,2
COMMENTS
When computing the cyclotomic polynomial Phi(n,x) as the quotient of sparse polynomials (see Arnold and Monagan), the divisors of n greater than phi(n)/2 are not required because only powers up to phi(n)/2 need to be computed; the remaining terms can be inferred because all cyclotomic polynomials are palindromic for n>1. This sequence grows slowly; k first occurs at A161507(k).
LINKS
Antti Karttunen, Table of n, a(n) for n = 1..65537
Andrew Arnold and Michael Monagan Calculating cyclotomic polynomials of very large height
MATHEMATICA
Table[d=Divisors[n]; e=EulerPhi[n]; Length[Select[d, #>e/2&]], {n, 100}]
PROG
(PARI) A161506(n) = { my(p2 = eulerphi(n)); sumdiv(n, d, ((2*d)>p2)); }; \\ Antti Karttunen, Jan 19 2020
CROSSREFS
KEYWORD
nonn
AUTHOR
T. D. Noe, Jun 17 2009
STATUS
approved