OFFSET
1,1
COMMENTS
Also, dimension of the space of weight 2n cuspidal newforms for Gamma_0(88). - N. J. A. Sloane, Nov 24 2016
First differences are 4,6,4,6,4,6.... Also values of k such that k^(10*n) mod 10 = 8*(n mod 2)+1. - Gary Detlefs, Jul 04 2014
In other words, numbers n such that n^(2+4*k) + 1 is divisible by 10, for k >= 0. - Altug Alkan, Mar 30 2016
The rational generating function, the periodic first differences and Greubel's closed form are an immediate consequence of the structure of formula given by [Martin]. - R. J. Mathar, Apr 09 2016
A quasipolynomial of order 2 and degree 1: a(n) = 5n - 3 if n is even and 5n - 2 if n is odd. - Charles R Greathouse IV, Nov 03 2021
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..10000
Greg Martin, Dimensions of the spaces of cusp forms and newforms on Gamma_0(N) and Gamma_1(N), J. Numb. Theory 112 (2005) 298-331, Theorem 1.
William A. Stein, Dimensions of the spaces S_k^{new}(Gamma_0(N)).
William A. Stein, The modular forms database.
Index entries for linear recurrences with constant coefficients, signature (1,1,-1).
FORMULA
a(n) = 4*floor(n/2) + 6*floor((n-1)/2) + 3. - Gary Detlefs, Jul 04 2014
G.f.: 3*x - x^2*(-7-6*x+3*x^2)/((1+x)*(x-1)^2). - R. J. Mathar, Jul 15 2015
From G. C. Greubel, Mar 30 2016: (Start)
a(n) = (1/2)*(10*n - 5 - (-1)^n).
E.g.f.: (5*x + 3)*cosh(x) + (5*x + 2)*sinh(x). (End)
Sum_{n>=1} (-1)^(n+1)/a(n) = sqrt(5-2*sqrt(5))*Pi/10. - Amiram Eldar, Sep 26 2022
MAPLE
# see A063195
MATHEMATICA
Table[4 Floor[n/2] + 6 Floor[(n - 1)/2] + 3, {n, 50}] (* or *)
Table[SeriesCoefficient[3 x - x^2 (-7 - 6 x + 3 x^2)/((1 + x) (x - 1)^2), {x, 0, n}], {n, 50}] (* Michael De Vlieger, Mar 30 2016 *)
LinearRecurrence[{1, 1, -1}, {3, 7, 13}, 100] (* G. C. Greubel, Mar 30 2016 *)
PROG
(PARI) my(x='x+O('x^99)); Vec(3*x-x^2*(-7-6*x+3*x^2)/((1+x)*(x-1)^2)) \\ Altug Alkan, Mar 31 2016
(PARI) a(n)=5*n-3+n%2 \\ Charles R Greathouse IV, Mar 31 2016
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
N. J. A. Sloane, Jul 10 2001
STATUS
approved