FACTDOUBLE
Returns the double factorial of a number: n × (n-2) × (n-4) × ... down to 1 or 2.
FACTDOUBLE(number)Arguments
numberThe non-negative integer for which to compute the double factorial.
=FACTDOUBLE(7)7 × 5 × 3 × 1 = 105 (odd: down to 1)
=FACTDOUBLE(6)6 × 4 × 2 = 48 (even: down to 2)
- •Returns #NUM! if number is negative
- •Returns 1 if number is 0
- •Used in combinatorics and special functions (e.g. Bessel)
- •Confusing FACTDOUBLE with a double application of FACT (like FACT(FACT(n))) - the double factorial is a completely different operation that multiplies only every other integer down to 1 or 2, not the factorial applied twice
- •Expecting FACTDOUBLE(0) to return 0 - by mathematical convention, the double factorial of 0 is defined as 1, similar to how FACT(0) also equals 1
- •Passing a negative number, which returns a #NUM! error since the double factorial is only defined for non-negative integers
FACTCalculates the standard factorial (multiplying every integer down to 1), a different and typically much larger result than FACTDOUBLE's every-other-integer product.COMBINUses standard factorials internally, a related but distinct combinatorial calculation from FACTDOUBLE's specialized double factorial.MULTINOMIALAlso builds on factorial calculations, though for a different combinatorial purpose than FACTDOUBLE.Is FACTDOUBLE the same as applying FACT twice?
No, that's a common misconception - the double factorial multiplies only every other integer (n × (n-2) × (n-4) ... down to 1 or 2), not the entire factorial calculated a second time.
What is FACTDOUBLE(0)?
1, by mathematical convention - similar to how the standard factorial of 0 is also defined as 1.
Why does FACTDOUBLE give different results for odd versus even numbers?
Odd numbers multiply down to 1 (like 7×5×3×1), while even numbers multiply down to 2 (like 6×4×2) - the stopping point depends on whether the starting number is odd or even, which naturally produces different sequences.
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