FIB Function (Gnumeric)

• Introduced: Gnumeric 1.1.9 release notes explicitly record this function being added.
• Supported in: Gnumeric 1.12.62; no retirement is established.
• Scope: Desktop Gnumeric with the Mathematics plugin. English comma-separated formulas are shown.
• Checked: October 14, 2026. Tables and the named advanced check were recalculated through 1.12.57 ssconvert. Current 1.12.62 source was inspected; the later release was not executed.

FIB returns the Fibonacci sequence indexed with F₁ = F₂ = 1 and Fₙ = Fₙ₋₁ + Fₙ₋₂. Use it to explore recurrences or modest sequence indices. It is not an arbitrary-precision integer calculator. Versioned function help.

=FIB(n)

Function implementation.

Ordinary results are rounded to twelve decimal places; tiny values use scientific notation. FIB results retain the native captured digits so approximation is visible. The eight reference rows were independently checked.

Enter indices 1 through 10 in A2:A11. In B2 enter =FIB(A2) and fill down. The sequence should be 1, 1, 2, 3, 5, 8, 13, 21, 34, 55. To compare the recurrence, enter =B3+B2 in C4 and fill down beside B4:B11. This layout is a reproducible worksheet exercise; the direct calls in the tables were the executed checks.

The indexing matters: some mathematical definitions include F₀ = 0, but Gnumeric rejects index zero. Shift your source indices deliberately rather than replacing every error with zero.

The executed check =FIB(20)-FIB(19)-FIB(18) returned 0, agreeing with 6765 − 4181 − 2584. This verifies the recurrence for that modest index.

The current implementation keeps integer values through index 46, then uses Binet’s expression involving powers of (1 + √5)/2 and (1 − √5)/2. At index 47 the native result was 2971215073.000005 instead of exact 2971215073. At 78 the error was 24. Rounding to an integer would hide the first discrepancy but would not fix the second. Exact large-integer work needs an independently verified integer method and a number representation capable of holding the answer.

Validate the index before generating a sequence. The tested 5.9 input became index 5, while 0.9 became zero and failed. A blank reference is treated as zero here. TRUE became 1. The high-index limitations above are actual 1.12.57 observations; the retained later-source branch supports the warning but does not establish identical later-build output.

F5 is an actually empty cell. Listed errors are native error results, not mathematical zeroes.

• Worksheet implementation

The retained workbook contained a deliberately stale SUM cache and an uncached SUM formula, both recalculated correctly. Native evidence covers the listed 1.12.57 cases; worksheet layouts and algebraic interpretations are independently specified explanations. Other locales, direct GUI entry and later-release output were not tested.

SUM, FACT.