Date: Sat, 24 Jan 1998 12:48:37 -0500

From: Alex Bogomolny

Bernd, hello:

It's a known dictum that the output is at best as good as input. So the first part of Netscape's claim is a complete nonsense. The problem is of course in that they return raw result of a decimal operation executed in binary arithmetic.

Assume A and B are exact numbers while a and b are their
respective approximations: |A - a| < e1 and |B - b| < e2.
In your case, A = 11.8, B = 5. a and b are their approximations
in, say, 64 bit binary. I do not know how numbers are represented in
a 64 bit PC. I would assume that 1/5 (or, probably, 13 bits) are
occupied by exponent, 1 goes for the sign, 50 remain for mantissa.
It's safe to assume that both errors e1 and e2 are close to 2^{-50}.

You want to estimate |A/B - a/b|. (A/B = 2.36 and a/b is what JS hands out to you.)

|A/B - a/b| = |b(A - a) - a(B - b)| / (Bb)

Approximately this is bounded from above by
|A - a|/B + |B - b|A/B^{2} = 2^{-50}*(.2 + 11.8/25) = 2^{-50}*.672
Which is approximately .672*10^{-15}. And this is only twice
as large as JS gave you. The reason is that binary arithmetic
might smartly use rounding instead of chopping. So that you'd
get e1 = e2 = 2^{-49} instead of 2^{-50}. Everything fits snugly.

A good calculator should probably detect such cases and try to make up for inaccuracy in the binary arithmetic.

Best regards,

Alexander Bogomolny

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