1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
|
! Copyright (C) 2022 J. K. Dewhurst, S. Sharma and E. K. U. Gross.
! This file is distributed under the terms of the GNU General Public License.
! See the file COPYING for license details.
elemental real(8) function factn2(n)
implicit none
! arguments
integer, intent(in) :: n
! local variables
integer i
real(8), parameter :: f(38)=[ &
1.d0, 2.d0, &
3.d0, 8.d0, &
15.d0, 48.d0, &
105.d0, 384.d0, &
945.d0, 3840.d0, &
10395.d0, 46080.d0, &
135135.d0, 645120.d0, &
2027025.d0, 10321920.d0, &
34459425.d0, 185794560.d0, &
654729075.d0, 3715891200.d0, &
13749310575.d0, 81749606400.d0, &
316234143225.d0, 1961990553600.d0, &
7905853580625.d0, 51011754393600.d0, &
213458046676875.d0, 1428329123020800.d0, &
6190283353629375.d0, 42849873690624000.d0, &
191898783962510625.d0, 1371195958099968000.d0, &
6332659870762850625.d0, 46620662575398912000.d0, &
221643095476699771875.d0, 1678343852714360832000.d0, &
8200794532637891559375.d0, 63777066403145711616000.d0]
if (n <= 1) then
factn2=1.d0
else if (n <= 38) then
factn2=f(n)
else
factn2=dble(n)
do i=1,n/2-1
factn2=factn2*dble(n-2*i)
end do
end if
end function
|
