820

820 = 2² x 5 x 41.

820 has two representations as a sum of two squares: 820 = 6² + 28² = 12² + 26².

820 is the 40th triangular number.

820 is a divisor of 81² - 1.

810 is 1111 in base 9.

Muhammad ibn Musa al-Khwarizmi presented the first systematic solution of linear and quadratic equations in the year 820 in The Compendious Book on Calculation by Completion and Balancing.

Source: WolframAlpha