bessi0.f90

FORTRANvisualfortran常用数值算法集及源码

FUNCTION bessi0(x)
REAL bessi0,x
REAL ax
SAVE p1,p2,p3,p4,p5,p6,p7,q1,q2,q3,q4,q5,q6,&
     q7,q8,q9
DATA p1,p2,p3,p4,p5,p6,p7/1.0d0,3.5156229d0,&
     3.0899424d0,1.2067492d0,0.2659732d0,0.360768d-1,&
	 0.45813d-2/
DATA q1,q2,q3,q4,q5,q6,q7,q8,q9/0.39894228d0,&
     0.1328592d-1,0.225319d-2,-0.157565d-2,&
	 0.916281d-2,-0.2057706d-1,0.2635537d-1,&
     -0.1647633d-1,0.392377d-2/
if (abs(x)<3.75) then
  y=(x/3.75)**2
  bessi0=p1+y*(p2+y*(p3+y*(p4+y*(p5+y*(p6+y*p7)))))
else
  ax=abs(x)
  y=3.75/ax
  bessi0=(exp(ax)/sqrt(ax))*(q1+y*(q2+y*(q3+y*(q4+y*&
         (q5+y*(q6+y*(q7+y*(q8+y*q9))))))))
endif
END FUNCTION bessi0