HP Fortran question: precision of double-precision?

[ Please note the Reply-To; I'm posting this for a friend ... ]

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In a fortran application program under HPUX 9.01, we have an

iteration loop that needs to converge to very tight tolerances.

The independent variable is of the order of 30 and the convergence

criteria needs to be about 1.D-11. The whole program has

declared all floating point variables to be double precision so,

according to the HP Fortran manual, we should get over 15 significant

figures of accuracy. In checking this loop, however, it looks like

we're converging to about 9 significant digits and the remaining

6 are junk. As expected, the loop frequently does not converge.

So the questions are: Are we doing something wrong? Is HP Fortran

doing something wrong (it happens both with -O and -g turned on, so

it doesn't seem to be sensitive to optimization)? How can we find

out what's going on? Other pertinent information - we have both

the -K and +T switches turned on.

Thanks.

Bill Steer

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Thank you.

- Chuck

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