Square root of 2 the square root of two, with a reasonable small denominator, the fraction (≈ 1.4142857) is sometimes used. The sequence in the OEIS gives the numerical value for the square root of two, truncated to 65 decimal places: The Babylonian clay tablet YBC 7289 (c. 1800–1600 BC) gives an approximation of in four sexagesimal figures, , which is accurate to about six decimal digits, and is the closest possible three-place sexagesimal representation of : Another early close approximation is given in ancient Indian mathematical texts, the Sulbasutras (c. 800–200 BC) as follows: "Increase the length [of the side] by its third
Square root of the original estimate and its square. Furthermore, ("x" + "c") ≈ "x" + 2"xc" when "c" is close to 0, because the tangent line to the graph of "x" + 2"xc" + "c" at "c" = 0, as a function of "c" alone, is "y" = 2"xc" + "x". Thus, small adjustments to "x" can be planned out by setting 2"xc" to "a", or "c" = "a"/(2"x"). The most common iterative method of square root calculation by hand is known as the "Babylonian method" or "Heron's method" after the first-century Greek philosopher Heron of Alexandria, who first described it.