PlanMaker FACTDOUBLE: Double Factorials and Pairing Counts

• Introduced: Documented by the PlanMaker 2010 manual; the first introduction release is unknown.
• Availability: Listed in the 2026/NX English reference. No final supported-version endpoint is established.
• Scope: The checked desktop manuals do not establish identical behavior in every older, mobile or NX subscription build.
• Checked: October 14, 2026, against the official FACTDOUBLE reference. Documentation and independent calculations; no native PlanMaker test.

=FACTDOUBLE(Number)

The single argument is required. The PlanMaker 2026 reference requires a nonnegative number and says that decimal places are truncated.

For a positive even integer, multiply the even numbers from 2 through that integer. For a positive odd integer, multiply the odd numbers from 1 through it. In mathematical notation this operation is written with two exclamation marks, such as 10!!. In a worksheet, type FACTDOUBLE rather than adding exclamation marks to a cell reference.

Do not extend the nonnegative domain by first truncating a negative fraction yourself. The reference’s input restriction applies to the supplied number. It does not document every validation-order detail or an exact error token for every invalid input.

Set A1:C1 to Input, Double factorial, and Product check. Enter the A values below. Put =FACTDOUBLE(A2) in B2 and fill through B8. Enter the listed independent products separately in C:

The 10 and 11 cases reproduce the vendor’s examples. Row 8 follows the explicitly documented 2026 truncation rule: 10.9 is treated as 10, so it uses the even sequence. It is not rounded to 11.

In D2 enter =B2=C2 and fill through D8. All seven comparisons should be TRUE for these independently calculated cases. A comparison checks the chosen inputs and formulas, not the reliability of arbitrarily large results.

FACT multiplies every positive integer up to the selected integer. In a separate empty block, enter 6 in F2 and 7 in F3. Put =FACT(F2) in G2, =FACTDOUBLE(F2) in H2 and =FACTDOUBLE(F2)*FACTDOUBLE(F2-1) in I2; fill G2:I2 to row 3.

For these positive integers, the even and odd factors partition the ordinary factorial: n! = n!! × (n−1)!!. The identity explains why the product in I matches G. For n=6, the factors 6×4×2 and 5×3×1 together include each integer from 1 to 6 exactly once.

The notation is also different from taking the factorial twice. Applying FACT to FACT(6) would ask for 720!, a vastly larger and unrelated quantity.

Suppose six distinct participants need to be divided into three unordered pairs. Fix one participant: there are five choices for that person’s partner. Among the remaining four participants, fix another: there are three choices for the next partner. The last two form the final pair.

The number of pairings is therefore 5×3×1 = 15, obtained with =FACTDOUBLE(5). The general model for a positive even participant count n uses FACTDOUBLE(n-1).

On a separate sheet, put participant counts 4, 6, 8 in A2:A4. In B2 enter =FACTDOUBLE(A2-1) and fill through B4. The expected pairing counts are 3, 15, 105. These count pair memberships only: swapping the two people within a pair or rearranging the list of pairs does not create a new pairing.

This model assumes all participants are distinct and every pairing is allowed. Restrictions, repeated objects or ordered pair positions change the counting problem. An odd participant count cannot be fully paired under this model, even though the resulting FACTDOUBLE formula might still produce a number.

• The answer seems too small: verify whether you wanted FACT, FACTDOUBLE or twice FACT. They are different operations.
• A decimal changes the sequence: the 2026 documentation specifies truncation. Keep integer counts explicit when parity is part of the model.
• Very large inputs: products grow rapidly. The cited short entry does not give a complete overflow threshold or guarantee that every large integer is represented exactly.
• Zero, text and blanks: zero is within the stated nonnegative domain, but the displayed product definitions do not explicitly demonstrate its result or every coercion case. The worked examples here use positive numeric inputs.

Use PlanMaker’s formula-entry guidance for local settings, and retain the original input beside the result. Rounding a displayed product cannot restore integer precision that a calculation has already lost.