mirror of
https://git.planet-casio.com/Lephenixnoir/OpenLibm.git
synced 2025-01-01 06:23:39 +01:00
57 lines
1.8 KiB
Fortran
57 lines
1.8 KiB
Fortran
*DECK DCBRT
|
|
DOUBLE PRECISION FUNCTION DCBRT (X)
|
|
C***BEGIN PROLOGUE DCBRT
|
|
C***PURPOSE Compute the cube root.
|
|
C***LIBRARY SLATEC (FNLIB)
|
|
C***CATEGORY C2
|
|
C***TYPE DOUBLE PRECISION (CBRT-S, DCBRT-D, CCBRT-C)
|
|
C***KEYWORDS CUBE ROOT, ELEMENTARY FUNCTIONS, FNLIB, ROOTS
|
|
C***AUTHOR Fullerton, W., (LANL)
|
|
C***DESCRIPTION
|
|
C
|
|
C DCBRT(X) calculates the double precision cube root for
|
|
C double precision argument X.
|
|
C
|
|
C***REFERENCES (NONE)
|
|
C***ROUTINES CALLED D1MACH, D9PAK, D9UPAK
|
|
C***REVISION HISTORY (YYMMDD)
|
|
C 770601 DATE WRITTEN
|
|
C 890531 Changed all specific intrinsics to generic. (WRB)
|
|
C 890531 REVISION DATE from Version 3.2
|
|
C 891214 Prologue converted to Version 4.0 format. (BAB)
|
|
C***END PROLOGUE DCBRT
|
|
DOUBLE PRECISION X, CBRT2(5), Y, CBRTSQ, D9PAK, D1MACH
|
|
SAVE CBRT2, NITER
|
|
DATA CBRT2(1) / 0.6299605249 4743658238 3605303639 11 D0 /
|
|
DATA CBRT2(2) / 0.7937005259 8409973737 5852819636 15 D0 /
|
|
DATA CBRT2(3) / 1.0 D0 /
|
|
DATA CBRT2(4) / 1.2599210498 9487316476 7210607278 23 D0 /
|
|
DATA CBRT2(5) / 1.5874010519 6819947475 1705639272 31 D0 /
|
|
DATA NITER / 0 /
|
|
C***FIRST EXECUTABLE STATEMENT DCBRT
|
|
IF (NITER.EQ.0) NITER = 1.443*LOG(-.106*LOG(0.1*REAL(D1MACH(3)))
|
|
1 ) + 1.0
|
|
C
|
|
DCBRT = 0.D0
|
|
IF (X.EQ.0.D0) RETURN
|
|
C
|
|
CALL D9UPAK (ABS(X), Y, N)
|
|
IXPNT = N/3
|
|
IREM = N - 3*IXPNT + 3
|
|
C
|
|
C THE APPROXIMATION BELOW IS A GENERALIZED CHEBYSHEV SERIES CONVERTED
|
|
C TO POLYNOMIAL FORM. THE APPROX IS NEARLY BEST IN THE SENSE OF
|
|
C RELATIVE ERROR WITH 4.085 DIGITS ACCURACY.
|
|
C
|
|
Z = Y
|
|
DCBRT = .439581E0 + Z*(.928549E0 + Z*(-.512653E0 + Z*.144586E0))
|
|
C
|
|
DO 10 ITER=1,NITER
|
|
CBRTSQ = DCBRT*DCBRT
|
|
DCBRT = DCBRT + (Y-DCBRT*CBRTSQ)/(3.D0*CBRTSQ)
|
|
10 CONTINUE
|
|
C
|
|
DCBRT = D9PAK (CBRT2(IREM)*SIGN(DCBRT,X), IXPNT)
|
|
RETURN
|
|
C
|
|
END
|