The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A094247 Expansion of (phi(-q^5)^2 - phi(-q)^2) / 4 in powers of q where phi() is a Ramanujan theta function. 3
 1, -1, 0, -1, 1, 0, 0, -1, 1, -1, 0, 0, 2, 0, 0, -1, 2, -1, 0, -1, 0, 0, 0, 0, 1, -2, 0, 0, 2, 0, 0, -1, 0, -2, 0, -1, 2, 0, 0, -1, 2, 0, 0, 0, 1, 0, 0, 0, 1, -1, 0, -2, 2, 0, 0, 0, 0, -2, 0, 0, 2, 0, 0, -1, 2, 0, 0, -2, 0, 0, 0, -1, 2, -2, 0, 0, 0, 0, 0, -1, 1, -2, 0, 0, 2, 0, 0, 0, 2, -1, 0, 0, 0, 0, 0, 0, 2, -1, 0, -1, 2, 0, 0, -2 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,13 COMMENTS Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). LINKS G. C. Greubel, Table of n, a(n) for n = 1..1000 S. Cooper, On Ramanujan's function k(q)=r(q)r^2(q^2), Ramanujan J., 20 (2009), 311-328; see p. 313 eq. (2.4). L.-C. Shen, On the additive formulas of the theta functions and a collection of Lambert series pertaining to the modular equations of degree 5, Trans. Amer. Math. Soc. 345 (1994), no. 1, 323-345; see p. 336 eq. (3.13), p. 338 eq. (3.21). Michael Somos, Introduction to Ramanujan theta functions Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA Expansion of q * f(q^5) * f(-q^20) * chi(-q) in powers of q where f() and chi() are Ramanujan theta functions. Expansion of eta(q) * eta(q^10)^3 / (eta(q^2) * eta(q^5)) in powers of q. Euler transform of period 10 sequence [-1, 0, -1, 0, 0, 0, -1, 0, -1, -2, ...]. a(n) is multiplicative with a(2^e) = -1 if e > 0. a(5^e) = 1, a(p^e) = e+1 if p == 1, 5 (mod 8), a(p^e) = (1 + (-1)^e) / 2 if p == 3, 7 (mod 8). G.f. is a period 1 Fourier series which satisfies f(-1 / (40 t)) = 4 (t/i) g(t) where q = exp(2 Pi i t) and g(t) is the g.f. for A214316. - Michael Somos, Jul 12 2012 G.f.: x * Product_{k>0} (1 - x^k) * (1 - x^(10*k))^3 / ((1 - x^(2*k)) * (1 - x^(5*k))). G.f.: Sum_{k>0} Kronecker( -100, k) * x^k / (1 + x^k) = Sum_{k>0} Kronecker( -25, k) * x^k * (1 - x^k)^2 / (1 - x^(4*k)). - Michael Somos, Jul 12 2012 a(n+1) = (-1)^n * A053694(n). a(4*n + 1) = A122190(n). a(4*n + 3) = 0. a(2*n) = - A053694(n). - Michael Somos, Jul 12 2012 EXAMPLE G.f. = q - q^2 - q^4 + q^5 - q^8 + q^9 - q^10 + 2*q^13 - q^16 + 2*q^17 - q^18 - q^20 + ... MATHEMATICA a[ n_] := SeriesCoefficient[ (EllipticTheta[ 4, 0, q^5]^2 - EllipticTheta[ 4, 0, q]^2)/4, {q, 0, n}]; (* Michael Somos, Jul 12 2012 *) a[ n_] := SeriesCoefficient[ q QPochhammer[ -q^5] QPochhammer[ q^20] QPochhammer[q, q^2], {q, 0, n}]; (* Michael Somos, Jul 12 2012 *) PROG (PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x + A) * eta(x^10 + A)^3 / (eta(x^2 + A) * eta(x^5 + A)), n))}; (PARI) {a(n) = if( n<1, 0, -(-1)^n * sumdiv( n, d, kronecker( -100, d)))}; /* Michael Somos, Aug 24 2006 */ CROSSREFS Cf. A053694, A122190, A214316. Sequence in context: A001899 A059882 A303206 * A053694 A085862 A257392 Adjacent sequences:  A094244 A094245 A094246 * A094248 A094249 A094250 KEYWORD sign,mult AUTHOR Michael Somos, Apr 24 2004 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified August 1 17:43 EDT 2021. Contains 346402 sequences. (Running on oeis4.)