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 A062570 a(n) = phi(2*n). 19
 1, 2, 2, 4, 4, 4, 6, 8, 6, 8, 10, 8, 12, 12, 8, 16, 16, 12, 18, 16, 12, 20, 22, 16, 20, 24, 18, 24, 28, 16, 30, 32, 20, 32, 24, 24, 36, 36, 24, 32, 40, 24, 42, 40, 24, 44, 46, 32, 42, 40, 32, 48, 52, 36, 40, 48, 36, 56, 58, 32, 60, 60, 36, 64, 48, 40, 66, 64, 44, 48, 70, 48, 72 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS a(n) is also the number of non-congruent solutions to x^2 - y^2 == 1 (mod n). - Yuval Dekel (dekelyuval(AT)hotmail.com), Sep 21 2003 a(n) is the size of a square companion matrix of the minimal cyclotomic polynomial of (-1)^(1/n). - Eric Desbiaux, Dec 08 2015 a(n) is the degree of the (2n)-th cyclotomic field Q(zeta_(2n)). Note that Q(zeta_n) = Q(zeta_(2n)) for odd n. - Jianing Song, May 17 2021 REFERENCES T. M. Apostol, Introduction to Analytic Number Theory, Springer-Verlag, 1976, p. 28. LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..1000 N. J. A. Sloane, Families of Essentially Identical Sequences, Mar 24 2021 (Includes this sequence). László Tóth, Counting solutions of quadratic congruences in several variables revisited, arXiv preprint arXiv:1404.4214 [math.NT], 2014. László Tóth, Counting Solutions of Quadratic Congruences in Several Variables Revisited, J. Int. Seq. 17 (2014), Article 14.11.6. Wikipedia, Ramanujan's sum. FORMULA a(n) = Sum_{d|n and d is odd} n/d*mu(d). Multiplicative with a(2^e) = 2^e and a(p^e) = p^e-p^(e-1), p>2. Dirichlet g.f.: zeta(s-1)/zeta(s)*2^s/(2^s-1). - Ralf Stephan, Jun 17 2007 a(n) = A000010(2*n). a(n) = phi(n)*(1+((n+1) mod 2)). - Gary Detlefs, Jul 13 2011 a(n) = A173557(n)*b(n) where b(n) = 1, 2, 1, 4, 1, 2, 1, 8, 3, 2, 1, 4, 1, 2, ... is the multiplicative function defined by b(p^e) = p^(e-1) if p<>2 and b(2^e)=2^e. b(n) = n/A204455(n). - R. J. Mathar, Jul 02 2013 a(n) = -c_{2n}(n) where c_q(n) is Ramanujan's sum. - Michael Somos, Aug 23 2013 a(n) = A055034(2*n), for n >= 2. - Wolfdieter Lang, Nov 30 2013 O.g.f.: Sum_{n >= 1} mu(2*n-1)*x^(2*n-1)/(1 - x^(2*n-1))^2. - Peter Bala, Mar 17 2019 a(n) = A000010(4*n)/2, for n > = 1 (see Apostol, Theorem 2.5, (b), p. 28). - Wolfdieter Lang, Nov 17 2019 a(n) = n - Sum_{d|n, n/d odd, d < n} a(d). - Ilya Gutkovskiy, May 30 2020 Dirichlet convolution of A000010 and A209229. - Werner Schulte, Jan 17 2021 From Richard L. Ollerton, May 07 2021: (Start) a(n) = Sum_{k=1..n} A209229(gcd(n,k)). a(n) = Sum_{k=1..n} A209229(n/gcd(n,k))*phi(gcd(n,k))/phi(n/gcd(n,k)). (End) Sum_{k=1..n} a(k) ~ c * n^2, where c = 4/Pi^2 = 0.405284... (A185199). - Amiram Eldar, Oct 22 2022 MAPLE [phi(2*n)\$n=1..80]; # Muniru A Asiru, Mar 18 2019 MATHEMATICA Table[EulerPhi[2 n], {n, 80}] (* Vincenzo Librandi, Aug 23 2013 *) PROG (PARI) a(n) = eulerphi(2*n) (Sage) [euler_phi(2*n) for n in range(1, 74)] # Zerinvary Lajos, Jun 06 2009 CROSSREFS Cf. A000010, A000034, A008683, A037225, A060968, A062803, A185199, A209229. Sequence in context: A035114 A202103 A333787 * A108514 A317419 A120456 Adjacent sequences: A062567 A062568 A062569 * A062571 A062572 A062573 KEYWORD mult,nonn,easy AUTHOR Jason Earls, Jul 03 2001 EXTENSIONS Corrected by Vladeta Jovovic, Dec 04 2002 STATUS approved

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Last modified December 3 08:35 EST 2022. Contains 358515 sequences. (Running on oeis4.)