Work out double factorial instantly with clear inputs, formula shown and shareable results.
The double factorial n!! multiplies n by every second number down to 1 or 2, so it uses only the odd numbers when n is odd and only the even numbers when n is even. It satisfies n! = n!! × (n−1)!!.
Double factorial
n!! = n(n−2)(n−4)…, ending at 1 or 2, with 0!! = 1
9 × 7 × 5 × 3 × 1 = 945.
No, and the difference is enormous. 9!! = 945, whereas (9!)! is astronomically large.