double precision function dbesj0 (x)
c july 1977 edition.  w. fullerton, c3, los alamos scientific lab.
      double precision x, bj0cs(19), ampl, theta, xsml, y,
     1  d1mach, dcsevl, dcos, dsqrt
      external d1mach, dcos, dcsevl, dsqrt, initds
c
c series for bj0        on the interval  0.          to  1.60000e+01
c                                        with weighted error   4.39e-32
c                                         log weighted error  31.36
c                               significant figures required  31.21
c                                    decimal places required  32.00
c
      data bj0 cs(  1) / +.1002541619 6893913701 0731272640 74 d+0     /
      data bj0 cs(  2) / -.6652230077 6440513177 6787578311 24 d+0     /
      data bj0 cs(  3) / +.2489837034 9828131370 4604687266 80 d+0     /
      data bj0 cs(  4) / -.3325272317 0035769653 8843415038 54 d-1     /
      data bj0 cs(  5) / +.2311417930 4694015462 9049241177 29 d-2     /
      data bj0 cs(  6) / -.9911277419 9508092339 0485193365 49 d-4     /
      data bj0 cs(  7) / +.2891670864 3998808884 7339037470 78 d-5     /
      data bj0 cs(  8) / -.6121085866 3032635057 8184074815 16 d-7     /
      data bj0 cs(  9) / +.9838650793 8567841324 7687486364 15 d-9     /
      data bj0 cs( 10) / -.1242355159 7301765145 5158970068 36 d-10    /
      data bj0 cs( 11) / +.1265433630 2559045797 9158272103 63 d-12    /
      data bj0 cs( 12) / -.1061945649 5287244546 9148175129 59 d-14    /
      data bj0 cs( 13) / +.7470621075 8024567437 0989155840 00 d-17    /
      data bj0 cs( 14) / -.4469703227 4412780547 6270079999 99 d-19    /
      data bj0 cs( 15) / +.2302428158 4337436200 5230933333 33 d-21    /
      data bj0 cs( 16) / -.1031914479 4166698148 5226666666 66 d-23    /
      data bj0 cs( 17) / +.4060817827 4873322700 8000000000 00 d-26    /
      data bj0 cs( 18) / -.1414383600 5240913919 9999999999 99 d-28    /
      data bj0 cs( 19) / +.4391090549 6698880000 0000000000 00 d-31    /
c
      data ntj0, xsml / 0, 0.d0 /
c
      if (ntj0.ne.0) go to 10
      ntj0 = initds (bj0cs, 19, 0.1*sngl(d1mach(3)))
      xsml = dsqrt (4.0d0*d1mach(3))
c
 10   y = dabs(x)
      if (y.gt.4.0d0) go to 20
c
      dbesj0 = 1.0d0
      if (y.gt.xsml) dbesj0 = dcsevl (.125d0*y*y-1.d0, bj0cs, ntj0)
      return
c
 20   call d9b0mp (y, ampl, theta)
      dbesj0 = ampl * dcos(theta)
c
      return
      end