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50 lines
1.7 KiB
Fortran
50 lines
1.7 KiB
Fortran
*DECK DBESKS
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SUBROUTINE DBESKS (XNU, X, NIN, BK)
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C***BEGIN PROLOGUE DBESKS
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C***PURPOSE Compute a sequence of modified Bessel functions of the
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C third kind of fractional order.
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C***LIBRARY SLATEC (FNLIB)
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C***CATEGORY C10B3
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C***TYPE DOUBLE PRECISION (BESKS-S, DBESKS-D)
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C***KEYWORDS FNLIB, FRACTIONAL ORDER, MODIFIED BESSEL FUNCTION,
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C SEQUENCE OF BESSEL FUNCTIONS, SPECIAL FUNCTIONS,
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C THIRD KIND
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C***AUTHOR Fullerton, W., (LANL)
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C***DESCRIPTION
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C
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C DBESKS computes a sequence of modified Bessel functions of the third
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C kind of order XNU + I at X, where X .GT. 0, XNU lies in (-1,1),
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C and I = 0, 1, ... , NIN - 1, if NIN is positive and I = 0, 1, ... ,
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C NIN + 1, if NIN is negative. On return, the vector BK(.) contains
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C the results at X for order starting at XNU. XNU, X, and BK are
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C double precision. NIN is an integer.
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C
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C***REFERENCES (NONE)
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C***ROUTINES CALLED D1MACH, DBSKES, XERMSG
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C***REVISION HISTORY (YYMMDD)
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C 770601 DATE WRITTEN
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C 890531 Changed all specific intrinsics to generic. (WRB)
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C 890831 Modified array declarations. (WRB)
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C 890831 REVISION DATE from Version 3.2
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C 891214 Prologue converted to Version 4.0 format. (BAB)
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C 900315 CALLs to XERROR changed to CALLs to XERMSG. (THJ)
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C***END PROLOGUE DBESKS
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DOUBLE PRECISION XNU, X, BK(*), EXPXI, XMAX, D1MACH
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SAVE XMAX
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DATA XMAX / 0.D0 /
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C***FIRST EXECUTABLE STATEMENT DBESKS
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IF (XMAX.EQ.0.D0) XMAX = -LOG (D1MACH(1))
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C
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IF (X .GT. XMAX) CALL XERMSG ('SLATEC', 'DBESKS',
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+ 'X SO BIG BESSEL K UNDERFLOWS', 1, 2)
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C
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CALL DBSKES (XNU, X, NIN, BK)
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C
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EXPXI = EXP (-X)
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N = ABS (NIN)
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DO 20 I=1,N
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BK(I) = EXPXI * BK(I)
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20 CONTINUE
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C
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RETURN
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END
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