source:
LMDZ4/tags/LMDZ4_V3_4/libf/dyn3dpar/spline.F
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1 | ! |
2 | ! $Header$ |
3 | ! |
4 | subroutine spline(x,y,n,yp1,ypn,y2) |
5 | |
6 | c |
7 | |
8 | c Routine to set up the interpolating function for a cubic spline |
9 | |
10 | c interpolation (see "Numerical Recipes" for details). |
11 | |
12 | c |
13 | implicit real (a-h,o-z) |
14 | implicit integer (i-n) |
15 | |
16 | parameter(nllm=4096) |
17 | |
18 | dimension x(n),y(n),y2(n),u(nllm) |
19 | |
20 | c |
21 | c write(6,*)(x(i),i=1,n) |
22 | c write(6,*)(y(i),i=1,n) |
23 | |
24 | if(yp1.gt.0.99E30) then |
25 | c the lower boundary condition is set |
26 | y2(1)=0. |
27 | c either to be "natural" |
28 | u(1)=0. |
29 | |
30 | else |
31 | c or else to have a specified first |
32 | y2(1)=-0.5 |
33 | c derivative |
34 | u(1)=(3./(x(2)-x(1)))*((y(2)-y(1))/(x(2)-x(1))-yp1) |
35 | |
36 | end if |
37 | |
38 | do 11 i=2,n-1 |
39 | c decomposition loop of the tridiagonal |
40 | sig=(x(i)-x(i-1))/(x(i+1)-x(i-1)) |
41 | c algorithm. Y2 and U are used |
42 | p=sig*y2(i-1)+2. |
43 | c for temporary storage of the decompo- |
44 | y2(i)=(sig-1.)/p |
45 | c sed factors |
46 | u(i)=(6.*((y(i+1)-y(i))/(x(i+1)-x(i))-(y(i)-y(i-1)) |
47 | |
48 | . /(x(i)-x(i-1)))/(x(i+1)-x(i-1))-sig*u(i-1))/p |
49 | |
50 | 11 continue |
51 | |
52 | if(ypn.gt.0.99E30) then |
53 | c the upper boundary condition is set |
54 | qn=0. |
55 | c either to be "natural" |
56 | un=0. |
57 | |
58 | else |
59 | c or else to have a specified first |
60 | qn=0.5 |
61 | c derivative |
62 | un=(3./(x(n)-x(n-1)))*(ypn-(y(n)-y(n-1))/(x(n)-x(n-1))) |
63 | |
64 | end if |
65 | |
66 | y2(n)=(un-qn*u(n-1))/(qn*y2(n-1)+1.) |
67 | |
68 | do 12 k=n-1,1,-1 |
69 | c this is the backsubstitution loop of |
70 | y2(k)=y2(k)*y2(k+1)+u(k) |
71 | c the tridiagonal algorithm |
72 | 12 continue |
73 | |
74 | c |
75 | |
76 | return |
77 | |
78 | end |
79 |
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