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 A020500 Cyclotomic polynomials at x=1. 20
 0, 2, 3, 2, 5, 1, 7, 2, 3, 1, 11, 1, 13, 1, 1, 2, 17, 1, 19, 1, 1, 1, 23, 1, 5, 1, 3, 1, 29, 1, 31, 2, 1, 1, 1, 1, 37, 1, 1, 1, 41, 1, 43, 1, 1, 1, 47, 1, 7, 1, 1, 1, 53, 1, 1, 1, 1, 1, 59, 1, 61, 1, 1, 2, 1, 1, 67, 1, 1, 1, 71, 1, 73, 1, 1, 1, 1, 1, 79, 1, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Also the greatest common divisor of the prime factors of n. - Peter Luschny, Mar 22 2011 LINKS T. D. Noe, Table of n, a(n) for n = 1..10000 Yves Gallot, Cyclotomic polynomials and prime numbers Bartlomiej Bzdega, Andres Herrera-Poyatos, Pieter Moree, Cyclotomic polynomials at roots of unity, arXiv:1611.06783 [math.NT], 2016. See Lemma 19. FORMULA a(1) = 0; for n > 1, a(n) = gcd(lpf(n),gpf(n)), by Gallot's theorem 1.4. - Thomas Ordowski, May 04 2013 For n > 2, a(n) = lcm(1,2,...,n)/lcm(1,...,n-1). - Thomas Ordowski, Nov 01 2013 MAPLE with(numtheory, cyclotomic); f := n->subs(x=1, cyclotomic(n, x)); seq(f(i), i=0..64); A020500 := n -> igcd(op(numtheory[factorset](n))): seq(A020500(i), i=1..73); # Peter Luschny, Mar 22 2011 MATHEMATICA Table[ Cyclotomic[n, 1], {n, 1, 73}] (* Jean-François Alcover, Jan 10 2013 *) Join[{0}, Table[GCD@@FactorInteger[n][[All, 1]], {n, 2, 80}]] (* Harvey P. Dale, Jul 18 2019 *) PROG (PARI) a(n) = polcyclo(n, 1); \\ Michel Marcus, Oct 23 2015 (PARI) a(n) = if (n==1, 0, if (isprimepower(n, &p), p, 1)); \\ Michel Marcus, Nov 23 2016 CROSSREFS Apart from initial zero, same as A014963. Cf. A007947. Sequence in context: A276835 A349630 A157753 * A014963 A099636 A099635 Adjacent sequences:  A020497 A020498 A020499 * A020501 A020502 A020503 KEYWORD nonn,easy,nice AUTHOR STATUS approved

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Last modified May 16 05:56 EDT 2022. Contains 353693 sequences. (Running on oeis4.)