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 A028928 Theta series of quadratic form (or lattice) with Gram matrix [ 3, 1; 1, 5 ]. 3
 1, 0, 0, 2, 0, 2, 2, 0, 0, 0, 2, 0, 2, 2, 0, 0, 0, 0, 0, 2, 2, 2, 0, 0, 2, 0, 2, 4, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 2, 0, 2, 0, 2, 0, 0, 6, 0, 0, 2, 0, 0, 0, 2, 0, 4, 0, 0, 0, 0, 2, 0, 2, 0, 0, 0, 0, 0, 0, 0, 4, 2, 0, 0, 0, 0, 6, 2, 0, 0, 0, 2, 0, 0, 2, 2, 0, 0, 0, 0, 0, 6, 2, 0, 0, 0, 0, 2, 0, 0 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,4 COMMENTS Ramanujan theta functions: f(q) (see A121373), phi(q) (A000122), psi(q) (A010054), chi(q) (A000700). The number of integer solutions (x, y) to n = 3*x^2 + 2*x*y + 5*y^2, discriminant -56. - Ray Chandler, Jul 12 2014 LINKS John Cannon, Table of n, a(n) for n = 0..10000 Michael Gilleland, Some Self-Similar Integer Sequences N. J. A. Sloane et al., Binary Quadratic Forms and OEIS (Index to related sequences, programs, references) Michael Somos, Introduction to Ramanujan theta functions Eric Weisstein's World of Mathematics, Ramanujan Theta Functions FORMULA Expansion of phi(q^3) * phi(q^42) + 2*q^5 * chi(q) * psi(-q^3) * chi(q^14) * psi(-q^42) = phi(q^6) * phi(q^21) + 2*q^3 * chi(q^2) * psi(-q^6) * chi(q^7) * psi(-q^21) = phi(q^2) * phi(q^7) - 2*q^2 * phi(-q^4) * psi(q^7) * chi(-q) / chi(-q^28) in powers of q where phi(), psi(), chi() are Ramanujan theta functions - Michael Somos and Alex Berkovich, Jun 06 2011 Expansion of - phi(q) * phi(q^14) + 2 * chi(q) * f(-q^7) * f(-q^8) * chi(q^14) in powers of q where phi(), chi(), f() are Ramanujan theta functions - Michael Somos, Jun 22 2011 G.f.: Sum_{n, m in Z} x^(3*n*n + 2*n*m + 5*m*m). EXAMPLE G.f. = 1 + 2*q^3 + 2*q^5 + 2*q^6 + 2*q^10 + 2*q^12 + 2*q^13 + 2*q^19 + 2*q^20 + 2*q^21 + 2*q^24 + 2*q^26 + 4*q^27 + 2*q^35 + 2*q^38 + 2*q^40 + 2*q^42 + 6*q^45 + ... MATHEMATICA a[ n_] := If[ n < 1, Boole[ n == 0], If[ -1 != KroneckerSymbol[ -7, n / 7^IntegerExponent[ n, 7]], 0, Sum[ KroneckerSymbol[ -14, d], { d, Divisors @ n}]]]; (* Michael Somos, Jul 13 2011 *) PROG (PARI) {a(n) = if( n<1, n==0, qfrep([3, 1; 1, 5], n)[n] * 2)}; /* Michael Somos, Jun 06 2011 */ (PARI) {a(n) = if( n<1, n==0, (-1 == kronecker( -7, n / 7^valuation( n, 7))) * sumdiv( n, d, kronecker( -14, d)))}; /* Michael Somos, Jun 22 2011 */ CROSSREFS Cf. A106915. Sequence in context: A298141 A160210 A174610 * A260149 A091379 A151758 Adjacent sequences:  A028925 A028926 A028927 * A028929 A028930 A028931 KEYWORD nonn AUTHOR STATUS approved

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Last modified April 16 07:59 EDT 2021. Contains 343030 sequences. (Running on oeis4.)