OFFSET
0,4
COMMENTS
From Paul Curtz, Mar 26 2011: (Start)
p=0: 0, 1, 1, 9, 4, 25, 9, a(n),
p=1: 0, 1, 3, 6, 10, 15, 21, A000217,
p=2: 0, 3, 2, 15, 6, 35, 12, A142705,
p=3: 0, 2, 5, 9, 14, 20, 27, A000096,
p=4: 0, 5, 3, 21, 8, 45, 15, A171621,
p=5: 0, 3, 7, 12, 18, 25, 33, A055998,
p=6: 0, 7, 4, 27, 10, 55, 18,
p=7: 0, 4, 9, 15, 22, 30, 39, A055999,
p=8: 0, 9, 5, 33, 12, 65, 21, (see A061041),
p=9: 0, 5, 11, 18, 26, 35, 45, A056000. (End)
The moment generating function of p(x, m=2, n=1, mu=2) = 4*x*E(x, 2, 1), see A163931 and A274181, is given by M(a) = (-4 * log(1-a) - 4 * polylog(2, a))/a^2. The series expansion of M(a) leads to the sequence given above. - Johannes W. Meijer, Jul 03 2016
LINKS
G. C. Greubel, Table of n, a(n) for n = 0..1000
John M. Campbell, An Integral Representation of Kekulé Numbers, and Double Integrals Related to Smarandache Sequences, arXiv preprint arXiv:1105.3399 [math.GM], 2011.
Index entries for linear recurrences with constant coefficients, signature (0,3,0,-3,0,1).
FORMULA
From R. J. Mathar, Jan 22 2011: (Start)
G.f.: x*(1 + x + 6*x^2 + x^3 + x^4) / ((1-x)^3*(1+x)^3).
a(n) = 3*a(n-2) - 3*a(n-4) + a(n-6).
a(n) = n^2*(5 - 3*(-1)^n)/8. (End)
a(n) = A026741(n)^2.
a(n) - a(n-4) = 4*A064680(n+2). - Paul Curtz, Mar 27 2011
From Peter Bala, Feb 19 2019: (Start)
a(n) = numerator(n^2/(n^2 + 4)) = n^2/(gcd(n^2,4)) = (n/gcd(n,2))^2.
a(n) = n^2/b(n), where b(n) = [1, 4, 1, 4, ...] is a purely periodic sequence of period 2. Thus a(n) is a quasi-polynomial in n.
O.g.f.: x*(1 + x)/(1 - x)^3 - 3*x^2*(1 + x^2)/(1 - x^2)^3.
Cf. A181318. (End)
From Werner Schulte, Aug 30 2020: (Start)
Multiplicative with a(2^e) = 2^(2*e-2) for e > 0, and a(p^e) = p^(2*e) for prime p > 2.
Dirichlet g.f.: zeta(s-2) * (1 - 3/2^s).
Sum_{n>0} 1/a(n) = Pi^2 * 7 / 24. (End)
Sum_{k=1..n} a(k) ~ (5/24) * n^3. - Amiram Eldar, Nov 28 2022
MAPLE
a := proc(n): n^2*(5-3*(-1)^n)/8 end: seq(a(n), n=0..46); # Johannes W. Meijer, Jul 03 2016
MATHEMATICA
LinearRecurrence[{0, 3, 0, -3, 0, 1}, {0, 1, 1, 9, 4, 25}, 60] (* Harvey P. Dale, May 14 2011 *)
f[n_] := Numerator[(n/2)^2]; Array[f, 60, 0] (* Robert G. Wilson v, Dec 18 2012 *)
CoefficientList[Series[x(1+x+6x^2+x^3+x^4)/((1-x)^3(1+x)^3), {x, 0, 60}], x] (* Vincenzo Librandi, Jul 10 2016 *)
PROG
(PARI) concat(0, Vec(x*(1+x+6*x^2+x^3+x^4)/((1-x)^3*(1+x)^3) + O(x^60))) \\ Altug Alkan, Jul 04 2016
(PARI) a(n) = lcm(4, n^2)/4; \\ Andrew Howroyd, Jul 26 2018
(Magma) I:=[0, 1, 1, 9, 4, 25]; [n le 6 select I[n] else 3*Self(n-2)-3*Self(n-4)+Self(n-6): n in [1..60]]; // Vincenzo Librandi, Jul 10 2016
(Sage) (x*(1+x+6*x^2+x^3+x^4)/(1-x^2)^3).series(x, 60).coefficients(x, sparse=False) # G. C. Greubel, Feb 20 2019
CROSSREFS
KEYWORD
nonn,easy,mult
AUTHOR
Paul Curtz, Nov 18 2009
STATUS
approved