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It looks like you have to program it yourself. At a boundary x == 2.^n, the distance to the next machine real is either x * $MachineEpsilon or x * $MachineEpsilon / 2. The documentation for MantissaExponent ambiguously states that the mantissa will be "between 1/b and 1". It seems be the case that 1 / b <= mantissa < 1. nextafter[0., y_] := Sign[y] ...


Not as clean as J.M.'s method but this seems to give the same result: 0.1 ~RealDigits~ 2 ~FromDigits~ 2 3602879701896397/36028797018963968 Follow with Numerator and Denominator if needed.


SetPrecision[] does this: SetPrecision[0.1, ∞] 3602879701896397/36028797018963968


Look at LK4[{a, b, c, d}, I, 0, 0, 0, 0] What has happened is that the a in the argument {a, b, c, d} has been replaced by {1, 2, 3, 4} in the Sum[..., {a, 1, 4}] code in the definition of LK4. If you change the definition of LK4 to use a different iterator, you get consistent results: LK4[coeff_, tau_, xi1_, xi2_, x_, y_] := ...

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