A famous formula for π: π/4 = 1 − 1/3 + 1/5 − 1/7 + … This comes from a trigonometry formula: arctan(x) = x − x³/3 + x⁵/5 − x⁷/7 + … Putting x = 1 gives the series for π/4. This result was discovered independently by: Gottfried Wilhelm Leibniz (1673), James Gregory (1671), Nilakantha Somayaji (around 1500, in Tantrasangraha).