23 lines
837 B
Fortran
23 lines
837 B
Fortran
FUNCTION bessi1(x)
|
|
REAL bessi1,x
|
|
REAL ax
|
|
DOUBLE PRECISION p1,p2,p3,p4,p5,p6,p7,q1,q2,q3,q4,q5,q6,q7,q8,q9,y
|
|
SAVE p1,p2,p3,p4,p5,p6,p7,q1,q2,q3,q4,q5,q6,q7,q8,q9
|
|
DATA p1,p2,p3,p4,p5,p6,p7/0.5d0,0.87890594d0,0.51498869d0,
|
|
*0.15084934d0,0.2658733d-1,0.301532d-2,0.32411d-3/
|
|
DATA q1,q2,q3,q4,q5,q6,q7,q8,q9/0.39894228d0,-0.3988024d-1,
|
|
*-0.362018d-2,0.163801d-2,-0.1031555d-1,0.2282967d-1,-0.2895312d-1,
|
|
*0.1787654d-1,-0.420059d-2/
|
|
if (abs(x).lt.3.75) then
|
|
y=(x/3.75)**2
|
|
bessi1=x*(p1+y*(p2+y*(p3+y*(p4+y*(p5+y*(p6+y*p7))))))
|
|
else
|
|
ax=abs(x)
|
|
y=3.75/ax
|
|
bessi1=(exp(ax)/sqrt(ax))*(q1+y*(q2+y*(q3+y*(q4+y*(q5+y*(q6+y*
|
|
*(q7+y*(q8+y*q9))))))))
|
|
if(x.lt.0.)bessi1=-bessi1
|
|
endif
|
|
return
|
|
END
|