double factorial
product of all the integers from 1 up to the integral input of the function that have the same parity as this input
en-GB/ˈdʌbl fækˈtɔːɹɪəl/
en-US/ˈdʌbəl fækˈtoʊɹiəl/
product of all the integers from 1 up to the integral input of the function that have the same parity as this input