- 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.

> a = 1+ 2^(-100);

> DD(a);

1.0000000000000000000000000000007888609052210118054

> prec=50!;

> DD(a);

Warning: double rounding occurred on invoking the double-double rounding operator.

Try to increase the working precision.

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