Hi Ron,

Am Wed, 25 Apr 2018 09:58:09 -0700 (PDT) schrieb Ron George:

f(n) = k1 x Sigma [w(i) x exp(-n-i)/tau1] where i goes from 1 to n-1.

your formula:
=$B$4*SUMPRODUCT(D2*EXP(-1*(ROW(INDIRECT($A$2&":"&C2-1)))/$B$3))
^^^^^^

C2 = 1 and C2-1 = 0
That causes the error.
Try:
=$B$4*SUMPRODUCT(D2*EXP(-1*(ROW(INDIRECT($A$2&":"&C2)))/$B$3))


Regards
Claus B.
--
Windows10
Office 2016