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A140728 Expansion of (phi(-q^3) * phi(-q^5) - phi(-q) * phi(-q^15)) / 2 in powers of q where phi() is a Ramanujan theta function. 2
1, 0, -1, -1, -1, 0, 0, 2, 1, 0, 0, 1, 0, 0, 1, -3, -2, 0, 2, 1, 0, 0, -2, -2, 1, 0, -1, 0, 0, 0, 2, 4, 0, 0, 0, -1, 0, 0, 0, -2, 0, 0, 0, 0, -1, 0, -2, 3, 1, 0, 2, 0, -2, 0, 0, 0, -2, 0, 0, -1, 2, 0, 0, -5, 0, 0, 0, 2, 2, 0, 0, 2, 0, 0, -1, -2, 0, 0, 2, 3, 1, 0, -2, 0, 2, 0, 0, 0, 0, 0, 0, 2, -2, 0, -2, -4, 0, 0, 0, -1, 0, 0, 0, 0, 0 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,8

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..10000

M. Somos, Introduction to Ramanujan theta functions

Eric Weisstein's World of Mathematics, Ramanujan Theta Functions

FORMULA

Expansion of q * f(-q^2) * f(-q^30) * chi(-q^3) * chi(-q^5) in powers of q where f(), chi() are Ramanujan theta functions.

Expansion of eta(q^2) * eta(q^3) * eta(q^5) * eta(q^30) / (eta(q^6) * eta(q^10)) in powers of q.

Euler transform of period 30 sequence [0, -1, -1, -1, -1, -1, 0, -1, -1, -1, 0, -1, 0, -1, -2, -1, 0, -1, 0, -1, -1, -1, 0, -1, -1, -1, -1, -1, 0, -2, ...].

a(n) is multiplicative with a(2^e) = (-1)^e * (1-e) if e > 0. a(3^e) = a(5^e) = (-1)^e, a(p^e) = e+1 if p == 1, 4 (mod 15), a(p^e) = (-1)^e * (e+1) if p == 2, 8 (mod 15), a(p^e) = (1 + (-1)^e) / 2 if p == 7, 11, 13, 14 (mod 15).

G.f. is a period 1 Fourier series which satisfies f(-1 / (30 t)) = 60^(1/2) (t/i) g(t) where q = exp(2 Pi i t) and g() is the g.f. for A121362.

G.f.: x * Product_{k>0} (1 - x^(2*k)) * (1 - x^(30*k)) / ((1 + x^(3*k)) * (1 + x^(5*k))).

G.f.: Sum_{k>0} Kronecker(5, n) * x^n / (1 - x^n + x^(2*n)) = Sum_{k>0} -(-1)^n * Kronecker(5, n) * x^n / (1 + x^n + x^(2*n)).

a(n) = -(-1)^n * A140727(n). abs(a(n)) = A122855(n).

EXAMPLE

G.f. = q - q^3 - q^4 - q^5 + 2*q^8 + q^9 + q^12 + q^15 - 3*q^16 - 2*q^17 + ...

MATHEMATICA

a[ n_] := If[ n < 1, 0, DivisorSum[ n, -(-1)^# KroneckerSymbol[ 5, #] KroneckerSymbol[ -3, n/#] &]]; (* Michael Somos, Aug 26 2015 *)

a[ n_] := SeriesCoefficient[ q QPochhammer[ q^2] QPochhammer[ q^30] QPochhammer[ q^3, q^6] QPochhammer[ q^5, q^10], {q, 0, n}]; (* Michael Somos, Aug 26 2015 *)

a[ n_] := SeriesCoefficient[ (EllipticTheta[ 4, 0, q^3] EllipticTheta[ 4, 0, q^5] - EllipticTheta[ 4, 0, q] EllipticTheta[ 4, 0, q^15]) / 2, {q, 0, n}]; (* Michael Somos, Aug 26 2015 *)

PROG

(PARI) {a(n) = if( n<1, 0, sumdiv(n, d, -(-1)^d * kronecker(5, d) * kronecker(-3, n/d)))};

(PARI) {a(n) = my(A, p, e); if( n<1, 0, A = factor(n); prod( k=1, matsize(A)[1], [p, e] = A[k, ]; if( p==2, (-1)^e * (1-e), p==3 || p==5, (-1)^e, kronecker(p, 15)==1, (e+1) * (-1)^(e*valuation(p%15, 2)), (1 + (-1)^e) / 2)))};

(PARI) {a(n) = my(A); if( n<1, 0, n--; A = x * O(x^n); polcoeff( eta(x^2 + A) * eta(x^3 + A) * eta(x^5 + A) * eta(x^30 + A) / (eta(x^6 + A) * eta(x^10 + A)), n))};

CROSSREFS

Cf. A122855, A140727.

Sequence in context: A260649 A122855 A140727 * A254110 A298426 A130068

Adjacent sequences:  A140725 A140726 A140727 * A140729 A140730 A140731

KEYWORD

sign,mult,changed

AUTHOR

Michael Somos, May 29 2008

STATUS

approved

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Last modified November 14 02:19 EST 2019. Contains 329108 sequences. (Running on oeis4.)