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 A138505 Expansion of ((phi(q) * phi(-q^2)^2)^2 - 1) / 4 in powers of q where phi() is a Ramanujan theta function. 2
 1, -1, -8, -1, 26, 8, -48, -1, 73, -26, -120, 8, 170, 48, -208, -1, 290, -73, -360, -26, 384, 120, -528, 8, 651, -170, -656, 48, 842, 208, -960, -1, 960, -290, -1248, -73, 1370, 360, -1360, -26, 1682, -384, -1848, 120, 1898, 528, -2208, 8, 2353, -651, -2320 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 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 Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA a(n) is multiplicative with a(2^e) = -1 if e>0, a(p^e) = ((p^2)^(e+1) - 1) / (p^2 - 1) if p == 1 (mod 4), a(p^e) = ((-p^2)^(e+1) - 1) / ( -p^2 - 1) if p == 3 (mod 4). G.f.: Sum_{k>0} -(-1)^k * (2*k-1)^2 * x^(2*k-1) / (1 + x^(2*k-1)). a(2*n) = (-1)^n * a(n). 4 * a(n) = A138504(n) unless n=0. a(n) = -(-1)^n * A002173(n). - Michael Somos, Sep 25 2015 EXAMPLE G.f. = q - q^2 - 8*q^3 - q^4 + 26*q^5 + 8*q^6 - 48*q^7 - q^8 + 73*q^9 + ... MATHEMATICA a[ n_] := If[ n < 1, 0, - DivisorSum[ n, #^2 KroneckerSymbol[ -4, #] (-1)^(n/#) &]]; (* Michael Somos, Sep 25 2015 *) PROG (PARI) {a(n) = if( n<1, 0, sumdiv(n, d, d^2 * kronecker(-4, d) * -(-1)^(n/d)))}; (PARI) {a(n) = my(A); if( n<0, 0, A = x * O(x^n); polcoeff( ((eta(x^2 + A)^9 / (eta(x + A)^2 * eta(x^4 + A)^4))^2 - 1) / 4, n))}; CROSSREFS Cf. A002173, A138504. Sequence in context: A183892 A019432 A211796 * A002173 A050458 A125166 Adjacent sequences:  A138502 A138503 A138504 * A138506 A138507 A138508 KEYWORD sign,mult AUTHOR Michael Somos, Mar 21 2008 STATUS approved

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Last modified February 27 01:52 EST 2020. Contains 332299 sequences. (Running on oeis4.)