Assuming that G4 and/or H4 are calculated values, then look at =(G1-4) and
=(H1-4). I think that you will find that one or both is not equal to 4.

Note that the parentheses are required in these formulas, else Excel will
return zero if they are equal to 15 digits, even though it will calculate
with the full precision non-difference. Similarly, if the differences
between G1 and H1 and 4 are less than 1E14, then formatting the cell to
display more figures will not help, since Excel (as documented) will display
no more than 15 digits; the rest will arbitrarily be zero if you ask for more.

Jerry

"derekcowley" wrote:

when mixing data types: a real and a complex as in

=IMSUM(-G1/2,IMSQRT(H1))

if G1 and H1 are both exactly equal to 4, the answer whould be 0

see http://www.geocities.com/derekcowley/public/IMerror.xls
and look at location I1 (highlighted).