# Re: Square root of a negative rral value

*From*: Gordon Sande <g.sande@xxxxxxxxxxxxxxxx>*Date*: Thu, 06 Nov 2008 13:15:12 GMT

On 2008-11-06 00:37:50 -0400, Terence <tbwright@xxxxxxxxx> said:

Well, yes, there was a 100 by100 matrix inversion, and a triple matrix

multiplication, and values wandered from about 0.4 up to 10**11, so it

might not be considered to be a stable calculation.

But the input is real-world data.

I was limited to REAL*8.

I accept the good point that 0.0 is a better default than the SQRT of

the absolute value, since the sample standard deviations SHOULD be

positive.

Some folks would take the view that the negative square root is

a hint that there are problems to be solved. Not shoved under the rug

or papered over or forgotten or otherwise ignored.

Anyway it works now, but I wonder just how many more pre-IBM360

algorithms have real-time problmes?? My best guide to "practical

methods" is the UK 1961 National Physica Laboratory White Paper on

Modern Computing Methods - often meaning hand-cranked calculators!

Why not take a look at Ake Bjorck's SIAM book on Least Squares. It

is much much much more modern than NPL's book on Modern Computing

Methods published by HMSO in the 1960s. A lot of its stuff is based

on fixed point with double length accumulation, the fi_2 mode.

.

**References**:**Square root of a negative rral value***From:*Terence

**Re: Square root of a negative rral value***From:*Gordon Sande

**Re: Square root of a negative rral value***From:*Glen Herrmannsfeldt

**Re: Square root of a negative rral value***From:*Terence

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