Description:

• doubledouble is both a function and a constant.
• As a function, it rounds its argument to the nearest number that can be written as the sum of two double precision numbers.
• The algorithm used to compute doubledouble(x) is the following: let xh = double(x) and let xl = double(x - xh). Return the number xh + xl. Note that if the current precision is not sufficient to exactly represent xh + xl, a rounding will occur and the result of doubledouble(x) will be useless.
• As a constant, it symbolizes the double-double precision format. It is used in contexts when a precision format is necessary, e.g. in the commands round, roundcoefficients and implementpoly. See the corresponding help pages for examples.