**pmontgom@sdcrdcf.UUCP (Peter Montgomery)** (04/27/87)

The following bc program prints (comments added by me) 999999996999999999700000000001 (b = actual square root) 999999994000000008400000001802000000083999999999400000000001 (c = square) 999999996999999999699950000000 (d = computed square root) 50000001 (b-d = difference) The problem exists both on a VAX 780 using 4.2bsd and on a SUN 2/170 using SUN release 3.0. a = 10^10 b = 1 - 30*a - 30*a^2 + a^3 c = b^2 d = sqrt(c) b c d b-d quit -- Peter Montgomery {aero,allegra,bmcg,burdvax,hplabs,ihnp4, psivax,randvax,sdcsvax,trwrb}!sdcrdcf!pmontgom Don't blame me for the crowded freeways - I don't drive.

**mike@arizona.edu (Mike Coffin)** (04/27/87)

```
In article <4534@sdcrdcf.UUCP>, pmontgom@sdcrdcf.UUCP (Peter Montgomery)
gives an example of a bc program that produces an incorrect square root.
I do not know if this is related, but there is also bug in the division
routine of bc, at least on VAX running Berkley 4.2 and 4.3. I had
several examples of this once; this is the only one I can find at the
moment:
-------------------------------------------------------------------------
% bc
obase=16
ibase=16
a = 519E2FEA51A537AB0132DC6F3678E8539F1BEF64109473F752
b = A2F01268A40089
a
519E2FEA51A537AB0132DC6F3678E8539F1BEF64109473F752
b
A2F01268A40089
c = a/b
c
803BF0FC7753CC1552D0A09B286CAAEBE771
a-(b*c)
F530645557A101E4C95E9095BD9
obase=A
c
11170769584511689511742018590-12440481240754033
---------------------------------------------------------------------------
The variable c in the above transcript is obviously bogus; a mod b
cannot be larger than b. And, when printed in base 10, it contains a
minus-sign. BTW, the correct answer for c is
803BF0FC7753CC1552D0A0CE7125CE93A4C4 (base 16).
mike coffin
(mike@arizona.edu)
```