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Could anyone explain why the evaluation of this gives a weird error?

a = 1.11111111111111111111; Do[a = 2 a - a; a, 50] enter image description here

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    $\begingroup$ It's Precision[] and its loss due to subtractive cancellation. $\endgroup$
    – Michael E2
    Oct 5, 2020 at 0:43

1 Answer 1

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In the calculation of the Precision[] of 2 a - a, the error in 2 a and a are treated as independent. Therefore the error bounds, 2 Δa and Δa, are added. Thus the new error bound is estimated as 3 Δa, and the Precision[] is reduced by Log10[3] each iteration. Therefore we run out of Precision[] in Precision[a]/Log10[3] steps:

a = 1.11111111111111111111;
Precision[a]/Log10[3]
(*  42.014  *)

[The a in the image and in the supplied code seem different.]

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