# r9atn1.f

          FUNCTION R9ATN1 (X)
    C***BEGIN PROLOGUE  R9ATN1
    ....
    ....
    Warning:   this routine is not intended to be user-callable.
    ....
    ....
    C***SUBSIDIARY
    C***PURPOSE  Evaluate ATAN(X) from first order relative accuracy so that
    C            ATAN(X) = X + X**3*R9ATN1(X).
    C***LIBRARY   SLATEC (FNLIB)
    C***CATEGORY  C4A
    C***TYPE      SINGLE PRECISION (R9ATN1-S, D9ATN1-D)
    C***KEYWORDS  ARC TANGENT, ELEMENTARY FUNCTIONS, FIRST ORDER, FNLIB,
    C             TRIGONOMETRIC
    C***AUTHOR  Fullerton, W., (LANL)
    C***DESCRIPTION
    C
    C Evaluate  ATAN(X)  from first order, that is, evaluate
    C (ATAN(X)-X)/X**3  with relative error accuracy so that
    C        ATAN(X) = X + X**3*R9ATN1(X).
    C
    C Series for ATN1       on the interval  0.          to  1.00000D+00
    C                                        with weighted error   2.21E-17
    C                                         log weighted error  16.66
    C                               significant figures required  15.44
    C                                    decimal places required  17.32
    C
    C***REFERENCES  (NONE)
    C***ROUTINES CALLED  CSEVL, INITS, R1MACH, XERMSG
    C***REVISION HISTORY  (YYMMDD)
    C   780401  DATE WRITTEN
    C   890206  REVISION DATE from Version 3.2
    C   891214  Prologue converted to Version 4.0 format.  (BAB)
    C   900315  CALLs to XERROR changed to CALLs to XERMSG.  (THJ)
    C   900720  Routine changed from user-callable to subsidiary.  (WRB)
    C***END PROLOGUE  R9ATN1