double precision function dbesy0 (x)
c august 1980 edition.  w. fullerton, c3, los alamos scientific lab.
      double precision x, by0cs(19), ampl, theta, twodpi, xsml,
     1  y, alnhaf, d1mach, dcsevl, dbesj0, dlog, dsin, dsqrt
      external d1mach, dbesj0, dcsevl, dlog, dsin, dsqrt, initds
c
c series for by0        on the interval  0.          to  1.60000e+01
c                                        with weighted error   8.14e-32
c                                         log weighted error  31.09
c                               significant figures required  30.31
c                                    decimal places required  31.73
c
      data by0 cs(  1) / -.1127783939 2865573217 9398054602 8 d-1      /
      data by0 cs(  2) / -.1283452375 6042034604 8088453183 8 d+0      /
      data by0 cs(  3) / -.1043788479 9794249365 8176227661 8 d+0      /
      data by0 cs(  4) / +.2366274918 3969695409 2415926461 3 d-1      /
      data by0 cs(  5) / -.2090391647 7004862391 9622395034 2 d-2      /
      data by0 cs(  6) / +.1039754539 3905725209 9924657638 1 d-3      /
      data by0 cs(  7) / -.3369747162 4239720967 1877534503 7 d-5      /
      data by0 cs(  8) / +.7729384267 6706671585 2136721637 1 d-7      /
      data by0 cs(  9) / -.1324976772 6642595914 4347606896 4 d-8      /
      data by0 cs( 10) / +.1764823261 5404527921 0038936315 8 d-10     /
      data by0 cs( 11) / -.1881055071 5801962006 0282301206 9 d-12     /
      data by0 cs( 12) / +.1641865485 3661495027 9223718574 9 d-14     /
      data by0 cs( 13) / -.1195659438 6046060857 4599100672 0 d-16     /
      data by0 cs( 14) / +.7377296297 4401858424 9411242666 6 d-19     /
      data by0 cs( 15) / -.3906843476 7104373307 4090666666 6 d-21     /
      data by0 cs( 16) / +.1795503664 4361579498 2912000000 0 d-23     /
      data by0 cs( 17) / -.7229627125 4480104789 3333333333 3 d-26     /
      data by0 cs( 18) / +.2571727931 6351685973 3333333333 3 d-28     /
      data by0 cs( 19) / -.8141268814 1636949333 3333333333 3 d-31     /
c
      data twodpi / 0.6366197723 6758134307 5535053490 057 d0 /
      data alnhaf /-0.6931471805 5994530941 7232121458 18d0 /
      data nty0, xsml / 0, 0.d0 /
c
      if (nty0.ne.0) go to 10
      nty0 = initds (by0cs, 19, 0.1*sngl(d1mach(3)))
      xsml = dsqrt (4.0d0*d1mach(3))
c
 10   if (x.le.0.d0) call seteru (29hdbesy0  x is zero or negative, 29,
     1  1, 2)
      if (x.gt.4.0d0) go to 20
c
      y = 0.d0
      if (x.gt.xsml) y = x*x
      dbesy0 = twodpi*(alnhaf+dlog(x))*dbesj0(x) + .375d0
     1  + dcsevl (.125d0*y-1.d0, by0cs, nty0)
      return
c
 20   call d9b0mp (x, ampl, theta)
      dbesy0 = ampl * dsin(theta)
      return
c
      end