33 lines
841 B
Fortran
33 lines
841 B
Fortran
FUNCTION bessi(n,x)
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INTEGER n,IACC
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REAL bessi,x,BIGNO,BIGNI
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PARAMETER (IACC=40,BIGNO=1.0e10,BIGNI=1.0e-10)
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CU USES bessi0
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INTEGER j,m
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REAL bi,bim,bip,tox,bessi0
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if (n.lt.2) pause 'bad argument n in bessi'
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if (x.eq.0.) then
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bessi=0.
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else
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tox=2.0/abs(x)
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bip=0.0
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bi=1.0
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bessi=0.
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m=2*((n+int(sqrt(float(IACC*n)))))
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do 11 j=m,1,-1
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bim=bip+float(j)*tox*bi
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bip=bi
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bi=bim
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if (abs(bi).gt.BIGNO) then
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bessi=bessi*BIGNI
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bi=bi*BIGNI
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bip=bip*BIGNI
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endif
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if (j.eq.n) bessi=bip
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11 continue
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bessi=bessi*bessi0(x)/bi
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if (x.lt.0..and.mod(n,2).eq.1) bessi=-bessi
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endif
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return
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END
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