OPT_FLOAT_STRK_LKBK Function (Gnumeric)
Use Gnumeric OPT_FLOAT_STRK_LKBK with historical extrema, nonzero carry and a tested floating-strike lookback call example.
Verification status: See the evidence and test scope described in this guide. How formula examples are checked
OPT_FLOAT_STRK_LKBK calculates a theoretical European floating-strike lookback option value using the running extrema already observed. A call uses the favorable minimum; a put uses the favorable maximum. These extrema are model inputs supplied by the worksheet author, not values the function downloads or reconstructs from a price history.
Syntax and input meaning
=OPT_FLOAT_STRK_LKBK(call_put_flag,spot,spot_min,spot_max,time,rate,cost_of_carry,volatility)
All eight arguments are required. Use “c” or “p”. spot is the current price, spot_min and spot_max are extrema observed so far, time is remaining years, and rate, carry and volatility are annual decimal inputs. For a consistent observation history, the current spot lies between the supplied minimum and maximum.
The call branch reads spot_min as its relevant historical extremum; the put branch reads spot_max. Both arguments still occupy required positions in the interface. Do not swap them or use an extremum from an unrelated observation period. The ordinary model also requires positive prices, positive time and positive volatility.
Reproducible new-observation control
=OPT_FLOAT_STRK_LKBK("c",100,100,100,1,0.05,0.05,0.2)
The independent expected result is approximately 17.2168022374. Here the current price and both historical extrema are 100, so no earlier favorable movement is built into the input. The remaining one-year model has 5% discount rate, 5% carry and 20% volatility.
For a reusable row, put the eight arguments in A2:H2 and use =OPT_FLOAT_STRK_LKBK(A2,B2,C2,D2,E2,F2,G2,H2). Label the extrema as observed-so-far values. These are not minimum and maximum terminal-price forecasts.
Independent payoff reasoning
For the call starting at its historical minimum, the expiry payoff is terminal price minus the minimum price reached over the full observation interval. The expected value was independently calculated using the reflection-principle distribution of the running minimum of drifted Brownian motion, integrated numerically and discounted. This is a different computational route from the closed-form cumulative-normal expression in Gnumeric.
The control’s value exceeds the corresponding ordinary at-the-money European call value of about 10.4505835722 because the lookback holder benefits from a future lower strike. That relationship is a useful sanity check for this particular matching setup, not a universal comparison between arbitrary contracts with different histories.
Advanced use: preserving observation history
When rolling a model forward, update the running minimum with any newly observed lower price and update the running maximum with any newly observed higher price. Merely replacing spot while leaving an inconsistent history can make the worksheet describe an impossible path. Also reduce remaining time consistently; changing spot alone does not represent all effects of elapsed time.
The continuous-observation model represented by this formula need not equal the value of a contract monitored only at specified discrete dates. A worksheet of daily closing prices does not by itself establish continuous extrema. Record the observation convention alongside your inputs rather than silently equating the two.
Singularities, errors and limits
The implementation explicitly divides by cost_of_carry and has no separate zero-carry limiting branch. Therefore this guide uses nonzero carry and does not recommend substituting zero into the formula merely because zero is valid in some other option functions. Near-zero carry can also involve cancellation and deserves independent numerical checking.
An invalid call/put flag yields #NUM! in the inspected callback. Other invalid inputs can reach logarithms, powers or square roots without comprehensive economic validation. The native control covers one valid call case only; puts, arbitrary historical extrema, zero-boundary limits and extreme parameters are not separately certified.
This is a model-value tutorial with synthetic prices. It neither retrieves market data nor establishes that any particular traded lookback contract uses these exact assumptions.
Verification scope and sources
On October 17, 2026, isolated Gnumeric ssconvert 1.12.57 runs recalculated the literal controls, returning 17.216802237360863. Each case used its own workbook and output cell F1; the instructional cell layouts are not separate GUI tests. Exact input and output files were retained. Other boundaries are source-derived unless expressly tested. Official 1.12.57 source was retrieved October 17, 2026. Original introduction and last functioning release are not established; no GUI screenshot or execution of another release is claimed. The stated input domain and test scope should travel with any compatibility comparison.