e^(π i) = -1 (Euler identity) x^4 + (n x^2 + 2 x) x^2 y^2 + y^4 = (x^3 + y^2)^2 for x^2 - n y^2 = 1 sum_(k=1)^∞ q(k) z^k = product_(k=1)^∞(1 + z^k) - 1 for abs(z)<1 product_(k=1)^∞(1 + z^k) = product_(k=1)^∞ 1/(1 - z^(2 k - 1)) for abs(z)<1 product_(k=1)^∞(1 + z^k) = 1 + sum_(k=1)^∞ q(k) z^k for abs(z)<1