Under the assumption that the ternary form x² + 2y² + 5z² + xz represents all odd positive integers, we prove that a ternary quadratic form ax² + by² + cz² (a,b,c ∈ ℕ) represents all positive integers n ≡ 4(mod 8) if and only if it represents the eight integers 4,12,20,28,52,60,140 and 308.
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Let q ∈ ℂ satisfy |q| < 1. If $f(q) = ∑_{n=0}^{∞} f_n q^n$ we write $[f(q)]_n = f_n$. We prove a general product-to-sum formula which includes known formulae such as $[q∏_{k=1}^{∞} (1-q^{2k})^3 (1-q^{6k})^3 _n = ∑_{{(x_1,x_2)∈ ℤ^2 \atop x_1^2+3x_2^2=n}} 1/2(x_1^2-3x_2^2)$ and $[q∏_{k=1}^{∞}(1-q^{4k})^6]_n = ∑_{{(x_1,x_2)∈ ℤ^2 \atop x_1^2+4x_2^2=n}} 1/2(x_1^2-4x_2^2).
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